The effect of alcohol on subsequent sleep in healthy adults: A systematic review and meta-analysis

Table of Contents

Overall Summary

Overview

This meta-analysis of 27 studies found that low doses of alcohol (0.35 g/kg) delay REM sleep onset and reduce REM sleep duration (0.50 g/kg) in a dose-dependent manner. High doses of alcohol (0.85 g/kg) shorten sleep onset latency and latency to N3 sleep (0.95 g/kg), also in a dose-dependent manner. The study could not determine the effects of alcohol on total sleep time, sleep efficiency, or wake after sleep onset due to large uncertainty. Meta-regression analyses showed a significant negative relationship between alcohol dose and REM sleep duration, and a significant positive relationship between alcohol dose and sleep onset latency. No moderating effect of sex was observed on objective sleep outcomes.

Key Points

Clear and concise summary (written-content)
The abstract provides a clear and concise summary of the study's aims, methods, key findings, and implications, effectively summarizing the complex research.
Section: Abstract
Highlights clinical relevance (written-content)
The abstract effectively highlights the clinical relevance of the findings by emphasizing the negative impact of even low doses of alcohol on REM sleep.
Section: Abstract
Acknowledges limitations (written-content)
The abstract acknowledges limitations and suggests future research directions, enhancing transparency and guiding further investigations.
Section: Abstract
Specify number of studies in meta-analysis (written-content)
The abstract could be improved by explicitly stating the number of studies included in the meta-analysis to provide a more accurate representation of the evidence base.
Section: Abstract
Clarify dose-response direction (written-content)
The abstract could be more informative by clarifying the direction of the dose-response relationships for specific sleep parameters.
Section: Abstract
Specify timing of alcohol consumption (written-content)
The abstract would benefit from specifying the typical timing of alcohol consumption relative to bedtime in the included studies.
Section: Abstract
Clear context and relevance (written-content)
The introduction effectively establishes the context of the study by highlighting the prevalence of sleep insufficiency and the common use of alcohol as a sleep aid.
Section: Introduction
Explanation of biphasic effects (written-content)
The introduction provides a concise overview of the biphasic effects of alcohol, which is crucial for understanding the complex relationship between alcohol and sleep.
Section: Introduction
Clear statement of aims (written-content)
The introduction clearly states the aims of the systematic review and meta-analysis, providing a roadmap for the reader.
Section: Introduction
Specify sleep parameters (written-content)
The introduction could be enhanced by explicitly stating the specific sleep parameters that will be examined in the study.
Section: Introduction
Specify typical timing of alcohol consumption (written-content)
The introduction would benefit from briefly mentioning the typical timeframe of alcohol consumption relative to bedtime in the included studies.
Section: Introduction
Connect to previous research (written-content)
The introduction could be improved by explicitly connecting the study to previous research, highlighting its novel contribution.
Section: Introduction
Clear and systematic search strategy (written-content)
The Methods section clearly outlines the search strategy using the PICO framework, enhancing transparency and replicability.
Section: Methods
Well-defined inclusion/exclusion criteria (written-content)
The inclusion and exclusion criteria are well-defined and appropriate for the research question, ensuring that only relevant studies were included.
Section: Methods
Rigorous assessment of reporting quality (written-content)
The use of the Cochrane RoB2 tool for crossover trials is a strength, providing a structured approach for assessing the methodological quality of the included studies.
Section: Methods
Thorough data extraction (written-content)
The data extraction process is thorough, with two researchers independently coding data and resolving discrepancies, minimizing errors and enhancing reliability.
Section: Methods
Clear definitions of sleep outcomes (written-content)
The Methods section clearly defines the sleep outcomes of interest, ensuring consistency and comparability across studies.
Section: Methods
Specify full search strings (written-content)
The Methods section could be improved by explicitly stating the specific search terms used for each database to enhance transparency and replicability.
Section: Methods
Justify dose classification cutoffs (written-content)
The Methods section would benefit from explicitly stating the criteria used to classify studies as low, moderate, or high dose.
Section: Methods
Detail handling of missing data (written-content)
The Methods section could be enhanced by explicitly stating how missing data or variance was handled, beyond just excluding studies with missing variance.
Section: Methods
Specify software and package versions (written-content)
The Methods section would be more accurate by explicitly stating the specific software and package versions used for the meta-analysis.
Section: Methods
Define 'healthy adult population' (written-content)
The Methods section could be improved by explicitly stating the specific criteria used to define 'healthy adult population'.
Section: Methods
Clarity and organization of Table 1 (graphical-figure)
Table 1 is clearly labeled and easy to understand, with column headers that accurately describe the content.
Section: Methods
Completeness of information in Table 1 (graphical-figure)
Table 1 could be improved by including a brief explanation of the PICO framework and the reason for 'N/A' in the 'Comparison' column, as well as the search terms for the 'Outcome' component.
Section: Methods
Clear summary of study selection (written-content)
The Results section begins with a clear and concise summary of the study selection process, directly referencing the PRISMA flow diagram.
Section: Results
Detailed description of alcohol conditions (written-content)
The Results section provides a detailed description of the alcohol conditions, classifying them into low, moderate, and high dose categories, which is crucial for understanding the dose-response relationships.
Section: Results
Comprehensive assessment of reporting quality (written-content)
The Results section includes a comprehensive assessment of reporting quality using the Cochrane RoB2 tool, which is essential for evaluating the reliability of the meta-analysis.
Section: Results
Well-structured presentation of objective sleep outcomes (written-content)
The presentation of objective sleep outcomes is well-structured, with clear reporting of mean differences, confidence intervals, and prediction intervals.
Section: Results
Detailed meta-regression analyses (written-content)
The Results section includes a detailed description of the meta-regression analyses, which explore the impact of alcohol dose and timing on sleep outcomes.
Section: Results
Specify number of studies per outcome (written-content)
The Results section could be improved by explicitly stating the number of studies that contributed data to each specific sleep outcome.
Section: Results
Explicitly state direction of effect (written-content)
The Results section would be clearer by explicitly stating the direction of effect (increase or decrease) for each sleep parameter in the text.
Section: Results
Specify statistical test for meta-regression (written-content)
The Results section could be enhanced by explicitly stating the specific statistical test used for the meta-regression analyses.
Section: Results
Explain the 95% prediction interval (written-content)
The Results section would be more accurate by providing a brief explanation of the 95% prediction interval (PI) and its relevance to the findings.
Section: Results
Ensure consistent use of 'N3 sleep' (written-content)
The Results section could be improved by consistently using the term 'N3 sleep' instead of 'N3 and N4 sleep' when referring to the combined deep sleep stage.
Section: Results
Clarity and logical flow of Figure 1 (graphical-figure)
Figure 1 is clearly labeled and the flow of information is easy to follow, demonstrating adherence to PRISMA guidelines.
Section: Results
Caption descriptiveness of Figure 1 (graphical-figure)
Figure 1 could benefit from a more descriptive caption that explicitly states that the diagram depicts the study selection process.
Section: Results
Organization and clarity of Table 2 (graphical-figure)
Table 2 is well-organized and provides a comprehensive overview of the key characteristics of each included study.
Section: Results
Completeness and consistency of Table 2 (graphical-figure)
Table 2 is very dense and could be improved by providing more detail on the 'Undisclosed' values and including clear units in the 'Proximity to bedtime' column.
Section: Results
Clarity and visual presentation of Table 3 (graphical-figure)
Table 3 is well-organized and uses symbols effectively to indicate risk of bias, providing an objective evaluation of the methodological limitations of each included study.
Section: Results
Completeness of information in Table 3 (graphical-figure)
Table 3 could be improved by including more detail on the reason for the risk of bias within each domain and the rationale for the overall risk rating.
Section: Results
Clarity and organization of Figure 2 (graphical-figure)
The forest plot (Figure 2) is well-organized and visually clear, effectively presenting the meta-analysis results for a variety of sleep parameters.
Section: Results
Caption and figure complexity of Figure 2 (graphical-figure)
Figure 2 could be improved by having a less lengthy caption and by including heterogeneity statistics (e.g., I^2).
Section: Results
Clarity and visual presentation of Figure 3 (graphical-figure)
Figure 3 is well-organized and clearly presents the dose-response relationships for different sleep outcomes.
Section: Results
Caption descriptiveness of Figure 3 (graphical-figure)
Figure 3 could be improved by explicitly stating that the y-axis represents the mean difference between alcohol and control conditions and by specifying the type of statistical model used for the meta-regression.
Section: Results
Clear summary of main findings (written-content)
The Discussion section effectively summarizes the main findings of the study, clearly stating the impact of low and high doses of alcohol on REM sleep and sleep onset latency.
Section: Discussion
Explanation of underlying mechanisms (written-content)
The Discussion section provides a clear explanation of the potential mechanisms underlying the observed effects of alcohol on sleep, linking the findings to the actions of neurotransmitters.
Section: Discussion
Acknowledgment of limitations (written-content)
The Discussion section acknowledges the limitations of the study, enhancing the credibility of the research.
Section: Discussion
Connection to practical implications (written-content)
The Discussion section effectively connects the findings to practical implications, highlighting the negative impact of alcohol on REM sleep.
Section: Discussion
Suggestions for future research (written-content)
The Discussion section identifies areas for future research, providing a clear direction for future investigations.
Section: Discussion
Specify sleep parameters with uncertain effects (written-content)
The Discussion section could be improved by explicitly stating the specific sleep parameters for which the study could not determine the influence of alcohol due to large uncertainty.
Section: Discussion
Connect dose-dependent effects to overall conclusion (written-content)
The Discussion section would benefit from explicitly connecting the dose-dependent effects of alcohol on sleep onset latency and REM sleep to the overall conclusion about the inappropriateness of alcohol as a sleep aid.
Section: Discussion
Specify limitations related to time in bed (written-content)
The Discussion section could be enhanced by explicitly stating the specific limitations related to the lack of control for time in bed in many of the included studies.
Section: Discussion
Ensure consistent use of 'N3 sleep' (written-content)
The Discussion section would be more accurate by consistently using the term 'N3 sleep' instead of 'deep sleep' or 'N3 and N4 sleep'.
Section: Discussion
Explain 'homeostatic control' (written-content)
The Discussion section could be improved by providing a brief explanation of the term 'homeostatic control' when discussing the expression of N3 sleep.
Section: Discussion
Clarity and visual appeal of Figure 4 (graphical-figure)
Figure 4 is visually appealing and presents the key findings of the meta-regression in a clear, intuitive manner.
Section: Discussion
Completeness and detail of Figure 4 (graphical-figure)
Figure 4 could be improved by including the direction of change for each outcome and explicitly listing the dose thresholds.
Section: Discussion
Clear summary of key findings (written-content)
The conclusion effectively summarizes the key findings of the study, clearly stating the dose-dependent effects of alcohol on REM sleep and sleep onset latency.
Section: Conclusion
Highlights practical implications (written-content)
The conclusion builds upon the discussion section by highlighting the practical implications of the findings, raising concerns about the use of alcohol as a sleep aid.
Section: Conclusion
Acknowledges study limitations (written-content)
The conclusion acknowledges the limitations of the study, enhancing the credibility of the research.
Section: Conclusion
Provides direction for future research (written-content)
The conclusion provides a clear direction for future research, helping guide future studies in the field.
Section: Conclusion
Specify source of uncertainty in findings (written-content)
The conclusion could be improved by explicitly stating that the uncertainty regarding total sleep time, sleep efficiency, and wake after sleep onset was due to the wide 95% prediction intervals.
Section: Conclusion
Connect dose effects to overall recommendation (written-content)
The conclusion would benefit from explicitly linking the dose-dependent effects of alcohol on sleep onset latency and REM sleep to the overall recommendation against using alcohol as a sleep aid.
Section: Conclusion
Specify limitations related to time in bed (written-content)
The conclusion could be enhanced by explicitly stating the specific limitations related to the lack of control for time in bed in many of the included studies.
Section: Conclusion
Ensure consistent use of 'N3 sleep' (written-content)
The conclusion would be more accurate by consistently using the term 'N3 sleep' instead of 'deep sleep'.
Section: Conclusion
Clarify statistical vs clinical significance (written-content)
The conclusion could be improved by explicitly stating that the findings are based on statistical significance and do not necessarily imply clinical significance.
Section: Conclusion

Conclusion

This meta-analysis provides a valuable contribution to the understanding of alcohol's effects on sleep architecture, particularly highlighting the dose-dependent disruption of REM sleep and the limited benefits on sleep onset latency. The study's methodological rigor, including the use of the PICO framework, RoB2 tool, and meta-regression, strengthens the reliability of the findings. However, limitations such as the lack of standardized alcohol doses, limited data on timing, and the inability to determine the effects on total sleep time, sleep efficiency, and wake after sleep onset due to large uncertainty, temper the conclusions. Despite these limitations, the study effectively answers its research questions by quantifying the impact of alcohol on various sleep parameters and identifying specific dose thresholds. The findings are significant for the field, challenging the common belief that alcohol is a helpful sleep aid and providing evidence-based guidance against its use for this purpose. The study's call for further well-controlled trials and investigation of individual factors such as sex underscores the need for more comprehensive research in this area. The practical implications are clear: alcohol is not an effective sleep aid and can disrupt sleep, particularly REM sleep, even at low doses.

Section Analysis

Abstract

Key Aspects

Strengths

Suggestions for Improvement

Introduction

Key Aspects

Strengths

Suggestions for Improvement

Methods

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

Table 1 Search strategy.
First Reference in Text
The search strategy was structured using the PICO framework (Table 1).
Description
  • Key aspect of what is shown: Table 1 shows the authors' strategy for finding relevant research articles. It's organized using the PICO framework, which is a way to structure a research question to guide the search process. PICO stands for Population, Intervention, Comparison, and Outcome. In this table, the 'Population or Problem' column shows the search terms used to find articles about the specific group being studied (e.g., 'sleep' but excluding terms like 'dependence' or 'anxiety'). The 'Intervention' column lists the search terms related to the treatment or factor being investigated, which here are 'alcohol' or 'ethanol'. The 'Comparison' column is marked as 'N/A' (Not Applicable) because this meta-analysis is not comparing one intervention against another. Finally, the 'Outcome' column lists the search terms related to the result being measured, such as 'sleep quality' or 'sleep duration'. The search terms are combined using 'OR' within each component and 'AND' across the components. This structured approach helps to ensure that the authors searched for articles on a very specific topic.
Scientific Validity
  • Adherence to methodological standards: The use of the PICO framework is a scientifically sound approach for structuring search strategies in systematic reviews and meta-analyses. This framework helps to ensure that the search is comprehensive and focused on the specific research question. The table explicitly defines the search terms used for population and intervention, which enhances transparency and replicability.
  • Completeness of search terms: The table provides information about the terms used to exclude articles (e.g., 'NOT dependence,' 'NOT anxiety'), which is important for understanding the scope of the review. The lack of specific search terms for the 'Outcome' component reduces the replicability and transparency of the search strategy. Furthermore, the rationale behind the choice of specific search terms is not provided, leaving some ambiguity in the process.
Communication
  • Clarity and organization: The table is clearly labeled and easy to understand, with column headers that accurately describe the content. The use of a table format facilitates the organization of the search strategy components. The use of Boolean operators is also clearly highlighted.
  • Completeness of information: The table includes a PICO framework, which is not defined, though the PICO framework is described in the reference text. It would improve clarity to include a brief explanation of the PICO framework in the table's caption or in a footnote. While the inclusion of 'N/A' in the 'Comparison' column is correct given the lack of a direct comparator, it would improve communication to have a brief explanation or note as to why this column has N/A values. The table also does not include the search terms used for the 'Outcome' component, which would provide further transparency.
  • Effectiveness of the reference text: The reference text effectively points to the table and correctly highlights that the search strategy is structured using the PICO framework.

Results

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

Fig. 1. Preferred Reporting Items for Systematic Reviews and Meta Analyses flow...
Full Caption

Fig. 1. Preferred Reporting Items for Systematic Reviews and Meta Analyses flow diagram outlining the process for selection of studies.

First Reference in Text
Following the screening process, 27 studies were identified for inclusion in the review (Fig. 1).
Description
  • Key aspect of what is shown: Figure 1 is a flow diagram, which is a visual way to show a process. This specific diagram illustrates how the authors of the study selected research articles for their analysis. It starts with a large number of articles found in databases and then shows how the number of articles is reduced at each step until a final set of studies is selected. The process is shown using boxes and arrows, where each box represents a stage in the selection and the arrows indicate the flow between stages. The initial box shows the number of articles found using a search of several electronic databases. The next stage shows that the number of articles were reduced by removing duplicates. The remaining articles went through a screening process which resulted in more exclusions. Next, full text versions of the articles were assessed for eligibility. Finally, this results in the inclusion of articles in the review. This process is based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a set of standards used to ensure transparency in research reviews.
Scientific Validity
  • Adherence to methodological standards: The use of a PRISMA flow diagram demonstrates adherence to established methodological standards for systematic reviews and meta-analyses. This shows a clear and transparent process for study selection, which is essential for the scientific rigor of the study. The diagram shows the number of articles identified, screened, and included, which is important for assessing the comprehensiveness of the search strategy.
  • Transparency and completeness of information: The flow diagram provides key information regarding the study selection process, such as the number of articles excluded and the reasons for exclusion. This allows readers to evaluate the potential for selection bias and to assess the overall rigor of the review.
Communication
  • Overall clarity and logical flow: The figure is clearly labeled and placed in a logical position within the text. The flow of information is easy to follow and is clearly presented with a simple, logical layout. The inclusion of the number of records at each stage provides transparency. The use of consistent visual elements (boxes, arrows) aids in understanding.
  • Caption descriptiveness: The figure could benefit from a more descriptive caption. While the caption states it is a flow diagram and includes the name of the guideline followed (PRISMA), it does not explicitly state what the diagram is showing. A more informative caption would clarify that the diagram depicts the study selection process for the meta-analysis.
  • Effectiveness of the reference text: The reference text is effective in that it clearly points the reader to the relevant figure and highlights that the figure is a representation of the screening process. It also indicates the result of this process.
Table 2 Summary characteristics of the included studies.
First Reference in Text
The key characteristics of each study are presented in Table 2.
Description
  • Key aspect of what is shown: Table 2 provides an overview of all the studies that were included in the meta-analysis. For each study, the table lists several key details, including the number of participants (sample size), their average age, their average weight, how the alcohol was given (method of administration), the amount of alcohol given per unit of body weight (alcohol dose in g/kg), how close to bedtime the alcohol was given (proximity to bedtime), how sleep was measured (method of sleep measurement) and the sleep outcomes that were reported in the study. The table also shows a final column indicating if there were significant changes observed in the sleep outcomes of each individual study when compared to a control condition. The abbreviations used in the table are defined in a footnote at the bottom of the table (e.g., TST= total sleep time, SE= sleep efficiency, SOL= sleep onset latency).
Scientific Validity
  • Methodological transparency: The table provides a strong foundation for understanding the methodological heterogeneity of the included studies. This is important for interpreting the results of the meta-analysis and for assessing the generalizability of the findings. The inclusion of the alcohol dose in g/kg and the proximity to bedtime are critical details, given the research question focuses on the effect of alcohol on sleep.
  • Completeness of methodological information: The inclusion of the method of sleep measurement allows readers to assess the validity of the sleep outcomes across studies. However, the consistent use of 'Undisclosed' values for various columns makes it challenging to fully assess the methodological details of the individual studies. Furthermore, the table does not provide specific detail on the type of sleep outcomes reported (e.g., specific sleep stages), which could limit the meta-analysis by combining different outcomes.
Communication
  • Organization and clarity: The table is well-organized and provides a comprehensive overview of the key characteristics of each included study. The use of clear column headers and consistent formatting makes it easy to navigate and compare across studies. The abbreviations used are defined in a footnote which makes it more understandable to a reader. The table is placed logically within the results section.
  • Completeness and consistency: The table includes a large amount of information in a concise format. However, this results in a very dense table that can be difficult to navigate. The inclusion of 'Undisclosed' values in many columns does not provide a clear understanding of the study characteristics. Furthermore, the lack of clear units (e.g., minutes, hours) in the 'Proximity to bedtime' column may hinder accurate interpretation.
  • Effectiveness of the reference text: The reference text correctly indicates the role of the table, highlighting that it provides the key characteristics of each included study. This helps the reader to understand why the table is important and to find it in the document.
Table 3 Results of the cochrane risk of bias (RoB2) Tool.
First Reference in Text
A summary of the risk of bias assessment is displayed in Table 3.
Description
  • Key aspect of what is shown: Table 3 presents the results of a risk of bias assessment conducted on each study included in the meta-analysis. Risk of bias is a measure of how likely a study's results are to be accurate or reliable, given how the study was designed and conducted. The table uses the Cochrane Risk of Bias tool for crossover trials (RoB2), which is a standard tool used to assess risk of bias in research. The table has a column for each of the domains assessed by RoB2, which are different areas of study design that can introduce bias. These domains include the randomization process, period and carryover effects, deviations from the intended intervention, missing outcome data, measurement of outcomes, and selection of the reported result. For each domain, the table indicates the risk of bias for each study using symbols. A plus symbol (+) means a low risk of bias. An exclamation mark (!) indicates some concerns about the risk of bias. A red circle indicates a high risk of bias. The final column provides an overall risk of bias rating based on the assessment of the individual domains. The abbreviations used in the table are defined in a footnote at the bottom of the table.
Scientific Validity
  • Adherence to methodological standards: The use of the Cochrane RoB2 tool is a scientifically valid approach to assess the risk of bias in the included studies. This tool is widely recognized and recommended for systematic reviews and meta-analyses. This provides an objective evaluation of the methodological limitations of each included study.
  • Completeness of methodological details: The table provides an overview of the risk of bias across multiple domains. The use of symbols to indicate risk of bias allows for easy comparison between studies. However, the lack of detailed information for the 'overall risk' (i.e., what determines the overall rating for a given study) and for the individual domains may limit the ability of the reader to fully assess the methodological limitations of the included studies. The decision to not include more granular detail in this table may have been a limitation based on space, but should have been included in a supplemental file for complete transparency. The RoB2 is a tool to be used in conjunction with specific details, which are not always included in the table, so it is important that the authors document the details behind these ratings.
Communication
  • Clarity and visual presentation: The table is well-organized, with clear column headers that correspond to the domains of the RoB2 tool and a row for each included study. The use of symbols (+, !, and red circles) to indicate risk of bias is visually effective. The table is placed logically within the results section. The table uses an abbreviation 'DS', which is not defined, but is explained in the footnote of the table.
  • Completeness of information: The table is effective in summarizing the RoB2 results, but could be improved with the inclusion of more detail. For example, the table does not specify the reason for the risk of bias within each domain. This lack of detail makes it harder for readers to evaluate the specific methodological limitations of each study. The overall risk of bias is presented, but the rationale is not always apparent.
  • Effectiveness of the reference text: The reference text is clear and accurately points to the table, highlighting that it provides a summary of the risk of bias assessment.
Fig. 2. Forest plot displays the duration (min) of total sleep time, non-rapid...
Full Caption

Fig. 2. Forest plot displays the duration (min) of total sleep time, non-rapid eye movement (NREM) stage one (N1) sleep, NREM stage two (N2) sleep, NREM stage three (N3) sleep, rapid eye movement (REM) sleep, and wake after sleep onset; the proportion (%) of sleep efficiency, N1 sleep, N2 sleep, N3 sleep, and REM sleep; and the latency (min) to sleep onset, REM sleep onset, and N3 sleep onset.

First Reference in Text
Sleep onset latency was not different between the alcohol condition and the control condition (Fig. 2; mean difference = -2.0 min, 95%CI = -4.3 to 0.2 min, 95%PI = -11.6 to 7.5
Description
  • Key aspect of what is shown: Figure 2 is a forest plot, which is a type of graph that visually summarizes the results of multiple studies. In this case, it summarizes the results of multiple studies that investigated the effect of alcohol on different aspects of sleep. The forest plot presents each sleep outcome in a separate row. For each sleep outcome, there is a point representing the average difference in the sleep outcome between the alcohol condition and a control condition. The size of the point represents the statistical weight of each effect size. A line extends from either side of the point representing the confidence interval (CI), which is a measure of the uncertainty around the average difference. The figure also shows a dotted line, which represents the prediction interval (PI), which shows the range where the true effect might lie. The forest plot also includes a label for each outcome, as well as the number of effect sizes (k) and participants (n) that were included in the analysis of each outcome. A negative effect size indicates a reduction in the outcome with the alcohol condition when compared to the control, and a positive effect size indicates an increase in the outcome with the alcohol condition when compared to the control. The outcomes included in this forest plot are related to sleep duration (total sleep time, NREM stage 1, NREM stage 2, NREM stage 3, REM sleep, and wake after sleep onset), sleep efficiency (the proportion of sleep efficiency, N1 sleep, N2 sleep, N3 sleep, and REM sleep), and sleep latency (the time to sleep onset, REM sleep onset, and N3 sleep onset).
Scientific Validity
  • Appropriate visualization of data: The use of a forest plot is appropriate for visually presenting the results of a meta-analysis. The plot allows for the assessment of the effect size for each outcome, and the associated uncertainty (confidence intervals). The inclusion of prediction intervals is valuable for showing the range in which future studies might find an effect. The forest plot also allows for the comparison of effect sizes across different sleep parameters.
  • Completeness of statistical information: The forest plot provides a good summary of the data, but it could be enhanced by including additional information such as heterogeneity statistics (e.g., I^2). The absence of these statistics limits the ability to evaluate the consistency of effects across studies. The effect sizes are presented as the mean difference between alcohol and control, but it is not always clear from the plot if these are standardized mean differences. The authors describe the data as ‘pooled’ which does not always reflect a standardized mean difference, so it would improve clarity to be more specific in the interpretation.
Communication
  • Clarity and organization: The forest plot is well-organized and visually clear, effectively presenting the meta-analysis results for a variety of sleep parameters. The use of consistent formatting and labels makes the plot relatively easy to navigate and interpret. The inclusion of the mean difference, confidence interval (CI), and prediction interval (PI) for each outcome are clearly displayed.
  • Caption and figure complexity: The caption is comprehensive in that it describes the data presented in the forest plot. However, due to the amount of detail, the caption is lengthy and may be difficult for some readers to process. The figure includes 12 separate forest plots within a single graphic, which can make it overwhelming. While the individual plots are easy to understand, the number of plots combined may obscure some differences. The x-axis label is not clear across the different plots, particularly as some are minutes and others are proportions.
  • Effectiveness of the reference text: The reference text is effective in that it clearly points the reader to the relevant figure and highlights that the figure shows that sleep onset latency was not different between the conditions. However, the text only describes one of the many outcomes presented in the forest plot.
Fig. 3. Meta-analytic model accounting for the dose of alcohol (gokg ¹ ) with...
Full Caption

Fig. 3. Meta-analytic model accounting for the dose of alcohol (gokg ¹ ) with standard drink equivalents (x-axis) based on the assumption of one standard drink containing 10 g of alcohol [57] and a standard body mass of 62 kg [56].

First Reference in Text
For every 1 g kg ¹ increase in alcohol dose, sleep onset latency in the alcohol condition was shortened by 6.4 min (Fig. 3; 95%CI = -9.3 to 3.5 min, k = 30, n = 222, p = 0.004) compared to the control condition, with a significantly shorter sleep onset latency identified with a high dose of alcohol (Fig. 3; ~0.85 g.kg 1).
Description
  • Key aspect of what is shown: Figure 3 shows four graphs, each illustrating the relationship between the dose of alcohol and different sleep outcomes. The x-axis of each graph represents the dose of alcohol, measured both in grams of alcohol per kilogram of body weight (g/kg) and in the equivalent number of standard alcoholic drinks. This conversion to standard drinks assumes that a standard drink contains 10 grams of alcohol and that the average person weighs 62 kg. The y-axis of each graph represents the difference in a specific sleep outcome between the alcohol condition and the control condition. Each point on the graph represents an effect size from an included study. The size of each point corresponds to the statistical weight of the study. The solid line represents the estimated relationship based on the meta-regression model, and the shaded area represents the 95% confidence interval around this line, indicating the uncertainty around the estimated relationship. The four graphs represent the relationship between alcohol dose and the following: 1) the duration of REM sleep; 2) sleep onset latency; 3) REM sleep onset latency; and 4) latency to N3 sleep. The slope of the regression line indicates whether the effect of alcohol on the sleep outcome is positive or negative, with the steeper slopes indicating a stronger effect. For example, a negative slope indicates that the alcohol group had reduced values when compared to the control condition.
Scientific Validity
  • Appropriate statistical methods: The use of meta-regression is a scientifically valid approach for investigating the dose-response relationship between alcohol and sleep outcomes. The figure effectively presents the meta-regression models, including the estimated effect, confidence intervals, and the dose-response relationship. The inclusion of both g/kg and standard drink equivalents on the x-axis facilitates the practical interpretation of the results.
  • Completeness of statistical information: The figure is limited by the lack of detail regarding the specific meta-regression model used, including any assumptions about linearity and potential non-linear relationships. Additionally, the figure does not indicate the variance of the effect sizes, which would provide a more complete representation of the data. While it is helpful to convert the alcohol dose to standard drinks, the use of a single standard body mass may not reflect the characteristics of all participants in the included studies, which is a limitation.
Communication
  • Clarity and visual presentation: The figure is well-organized and clearly presents the dose-response relationships for different sleep outcomes. The use of scatter plots with regression lines effectively illustrates the trends. The inclusion of confidence intervals provides a visual representation of the uncertainty around the estimated effects. The figure also presents the data in a way that allows for a meaningful interpretation (i.e., dose-response relationship).
  • Caption descriptiveness: The caption is generally informative, but it could be improved by explicitly stating that the y-axis represents the mean difference between alcohol and control conditions. Furthermore, while the caption mentions the conversion to standard drinks, this information is not consistently labeled on the x-axis of all the panels, which may cause confusion. While the caption states 'meta-analytic model', it does not specify the type of statistical model used for the meta-regression, so more detail here would improve transparency.
  • Effectiveness of the reference text: The reference text is effective in that it clearly points the reader to the relevant figure and describes that there is a relationship between alcohol dose and sleep onset latency. The reference text also highlights the specific finding that the effect is significant with a high dose of alcohol and also provides the cut off for this finding. The text only describes one of the four dose response relationships presented in the figure, which may lead a reader to overlook the other results.

Discussion

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

Fig. 4. Summary of the meta-regression findings with the dose-thresholds for...
Full Caption

Fig. 4. Summary of the meta-regression findings with the dose-thresholds for rapid eye movement (REM) sleep onset latency (min), REM sleep duration (min), sleep onset latency (min), and latency to non-rapid eye movement stage three (N3) sleep modelled for a 62 kg individual.

First Reference in Text
The clearest effect of alcohol was on REM sleep, with a low dose of alcohol ( ≤ 0.50 g kg ¹ ) delaying the first occurrence of REM sleep and reducing the duration of REM sleep across the sleep opportunity.
Description
  • Key aspect of what is shown: Figure 4 provides a visual summary of the dose-response relationships between alcohol consumption and four specific sleep outcomes. It uses a simplified image of a person with arrows and text to indicate the dose thresholds at which specific changes in sleep occur. The figure is based on the meta-regression analysis and shows that a dose of approximately 0.35 g/kg of alcohol delays the onset of rapid eye movement (REM) sleep, while a dose of approximately 0.50 g/kg reduces the duration of REM sleep. The figure also shows that a dose of approximately 0.85 g/kg of alcohol shortens sleep onset latency (the time it takes to fall asleep) and that a dose of approximately 0.95 g/kg shortens the latency to non-rapid eye movement (NREM) stage three (N3) sleep, also known as slow wave sleep. The figure is based on a standard 62kg individual.
Scientific Validity
  • Accuracy and validity: The figure accurately reflects the main findings of the meta-regression, which is a scientifically valid approach to explore dose-response relationships. The dose thresholds are based on statistical analysis. The use of a standard 62 kg individual provides a reference point for interpreting the dose-response relationships.
  • Limitations of simplification: The figure is a simplification of complex statistical findings. The use of fixed cut off values for the dose response relationship does not reflect the variance or confidence intervals associated with each relationship. It is also not clear how these thresholds are determined (e.g., based on statistical significance). The figure only represents the dose thresholds, and not the dose-response relationship itself, which could limit the interpretation of the overall results. The figure is also limited to a 62 kg individual, so is not generalizable to individuals of varying mass.
Communication
  • Clarity and visual appeal: The figure is visually appealing and presents the key findings of the meta-regression in a clear, intuitive manner. The use of the human figure helps to contextualize the findings, making it more accessible to a broader audience. The text is clearly labeled, and the dose thresholds are highlighted.
  • Completeness and detail: The figure could be improved by including the direction of change for each outcome. While the figure is an effective summary, it is an oversimplification of the findings. For example, the range of dose effects are not described, nor are the confidence intervals of these effects shown. The values for the dose thresholds are also not explicitly listed in the figure, relying on the reader to interpret them based on the text or the image itself.
  • Effectiveness of the reference text: The reference text accurately identifies the main finding of the review related to REM sleep and points to the relevant figure. However, it does not mention the other effects summarized in the figure, which may limit the reader's understanding of the overall impact of alcohol on sleep.

Conclusion

Key Aspects

Strengths

Suggestions for Improvement

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