Study analysis · Nutrients · 2025
Your watch says you slept fine—but you feel like garbage. Here's why.
Drinking coffee or energy drinks before bed doesn't make you sleep less, but it makes you think you did—especially if it's an energy drink.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study watched what happened when people drank coffee or energy drinks before bed and asked them how they slept. It found that even though their watches didn't show they slept less, they felt like they slept worse. But we can't say the caffeine caused that feeling — maybe they were stressed or ate junk food, and that's what really made them feel tired.
What’s the bottom line?
Scientists gave young adults coffee or energy drinks with caffeine right before bed and checked if they slept worse—using watches and their own feelings.
How strong is this study?
The study tried hard to measure sleep with watches and surveys, but people picked their own drinks (coffee or energy drinks), and the group was tiny. That means we can't be sure the results are fair or true for everyone — it's like guessing the taste of pizza after trying one slice from one pizzeria.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
36 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=40)+3.6/20
- Follow-up+10/10
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 546 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
This design cannot establish causation — the findings describe an association, not a cause. This is an observational cohort study without randomization or a control group that isolates caffeine as the sole variable. While it compares caffeine intake to a reference period, participants self-selected their group, and many confounders (e.g., stress, diet, mental health) were not measured or controlled, making it impossible to rule out other causes for the observed changes in sleep perception.
No Conflicts
No conflicts of interest identified
No conflicts of interest or funding sources were disclosed in the study text; no industry ties or funder involvement were identified.
The study was conducted at LMU Hospital Munich and approved by its ethics committee, but no funding sources, author affiliations with industry, or conflicts of interest were disclosed. The absence of a COI or funding statement introduces uncertainty, but no evidence of bias or industry influence was found in the text.
Key takeaways
- 01
Watches showed no change in sleep time or deep sleep.
- 02
But 30% felt they slept worse after coffee, and 47% felt worse after energy drinks—even though their sleep didn't actually change on the watch.
- 03
Even if your body sleeps the same, you might feel like you slept poorly after caffeine—especially from energy drinks—without feeling tired the next day.
Surprising findings
- Caffeine at 3 hours before bed didn't reduce total sleep time or deep sleep in young adults.Most public health advice warns against caffeine after 2 PM, based on studies showing sleep disruption. This study contradicts that with objective data from wearables in a real-world setting.
- Wearable devices detected no change in heart rate or sleep architecture, despite participants reporting increased nighttime awakenings.If people felt they woke up more, you'd expect the watch to pick up movement or elevated heart rate—but it didn't. This suggests the perception was psychological, not physiological.
Practical takeaways
If you drink caffeine before bed and feel like you slept poorly, check your wearable data before assuming it's the caffeine.
This only applies to young, healthy adults with no sleep disorders. It doesn't mean caffeine is safe for teens, older adults, or those with anxiety.
medium confidenceWhy this study matters
Caffeine Doesn't Shorten Sleep—But It Ruins Your Feelings
Despite consuming 240 mg of caffeine (equivalent to 2–3 cups of coffee) just 3 hours before bed, participants' wearable trackers showed no significant change in total sleep time, deep sleep, or wakefulness. Yet, self-reported poor sleep jumped from 21.6% to 29.7% after coffee and 46.6% after energy drinks.
You might be blaming caffeine for your tiredness, but your body might be sleeping just fine—your brain is just convinced otherwise. This disconnect could explain why so many people feel sleep-deprived even when they’re getting enough rest.
Energy Drinks Are Worse Than Coffee—Even With Same Caffeine
Both coffee and energy drinks had 240 mg of caffeine, but energy drinks caused nearly double the drop in perceived sleep quality: 46.6% reported poor sleep vs. 29.7% for coffee. Researchers suggest non-caffeine ingredients like sugar or taurine may amplify anxiety or discomfort.
If you think 'it's just the caffeine,' this study proves the drink's other ingredients might be sabotaging your sleep mindset—even if your body doesn't care.
You Don't Feel Tired—Even When You Think You Slept Badly
Despite feeling like their sleep was worse, participants showed no increase in daytime sleepiness or use of sleep aids. Their daily functioning remained unchanged, even when PSQI scores indicated 'poor sleep quality.'
Your perception of sleep doesn't always match your actual performance. This challenges the idea that feeling tired = bad sleep—and suggests anxiety or expectation might be the real culprit.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists gave young adults coffee or energy drinks with caffeine right before bed and checked if they slept worse—using watches and their own feelings.
Research results
Watches showed no change in sleep time or deep sleep. But 30% felt they slept worse after coffee, and 47% felt worse after energy drinks—even though their sleep didn't actually change on the watch.
What this means - more context
Even if your body sleeps the same, you might feel like you slept poorly after caffeine—especially from energy drinks—without feeling tired the next day.
Assesses whether 240 mg of caffeine from coffee or energy drinks consumed 3 hours before bedtime affects sleep in young healthy adults, using both wearable devices and self-reports.
Objective sleep metrics (duration, deep/light sleep, wake time, heart rate) showed no significant changes after caffeine intake, but subjective sleep quality worsened significantly—especially after energy drinks. No increase in daytime sleepiness or medication use was observed.
Methods Used
40 young healthy adults in an observational cohort study; self-assigned to coffee or energy drink groups (240 mg caffeine, 3h before bed) for 3 days after a 3-day water baseline; sleep measured via Withings ScanWatch wearables and Pittsburgh Sleep Quality Index questionnaires.
Main Finding
No statistically significant change in objective sleep duration or architecture (p=0.183), but self-reported poor sleep increased from 21.6% to 29.7% after coffee and 46.6% after energy drinks, indicating a dissociation between objective and perceived sleep quality.
Confidence Level
Moderate; small sample (n=40), no randomization, short duration (7 days), exploratory design, but uses validated wearables and questionnaires with consistent measurements.
Study Flags
Red Flags
- •No randomization (self-assigned groups)
- •Small sample size (n=40)
- •Short intervention period (7 days)
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Caffeine at 3 hours before bed didn't reduce total sleep time or deep sleep in young adults.
Most public health advice warns against caffeine after 2 PM, based on studies showing sleep disruption. This study contradicts that with objective data from wearables in a real-world setting.
Practical Takeaways
If you drink caffeine before bed and feel like you slept poorly, check your wearable data before assuming it's the caffeine.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 546 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
Human Cohort Study
Subject
Moderate probability
on the GRADE evidence scale
This study watched what happened when people drank coffee or energy drinks before bed and asked them how they slept. It found that even though their watches didn't show they slept less, they felt like they slept worse. But we can't say the caffeine caused that feeling — maybe they were stressed or ate junk food, and that's what really made them feel tired.
No conflicts of interest were detected in this study. No score impact.
Strengths
- Used validated tools (PSQI) and wearable devices for objective and subjective data
- Controlled caffeine dose and timing (240 mg, 3 hours before bed)
- Included a reference period (water) to adjust for fluid intake effects
Weaknesses
- No randomization — participants chose their group (coffee or energy drink)
- No true control group (e.g., placebo or no caffeine)
- Small sample size with no power calculation
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists gave young adults coffee or energy drinks with caffeine right before bed and checked if they slept worse—using watches and their own feelings.
Research results
Watches showed no change in sleep time or deep sleep. But 30% felt they slept worse after coffee, and 47% felt worse after energy drinks—even though their sleep didn't actually change on the watch.
What this means - more context
Even if your body sleeps the same, you might feel like you slept poorly after caffeine—especially from energy drinks—without feeling tired the next day.
Assesses whether 240 mg of caffeine from coffee or energy drinks consumed 3 hours before bedtime affects sleep in young healthy adults, using both wearable devices and self-reports.
Objective sleep metrics (duration, deep/light sleep, wake time, heart rate) showed no significant changes after caffeine intake, but subjective sleep quality worsened significantly—especially after energy drinks. No increase in daytime sleepiness or medication use was observed.
Methods Used
40 young healthy adults in an observational cohort study; self-assigned to coffee or energy drink groups (240 mg caffeine, 3h before bed) for 3 days after a 3-day water baseline; sleep measured via Withings ScanWatch wearables and Pittsburgh Sleep Quality Index questionnaires.
Main Finding
No statistically significant change in objective sleep duration or architecture (p=0.183), but self-reported poor sleep increased from 21.6% to 29.7% after coffee and 46.6% after energy drinks, indicating a dissociation between objective and perceived sleep quality.
Confidence Level
Moderate; small sample (n=40), no randomization, short duration (7 days), exploratory design, but uses validated wearables and questionnaires with consistent measurements.
Study Flags
Red Flags
- •No randomization (self-assigned groups)
- •Small sample size (n=40)
- •Short intervention period (7 days)
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Caffeine at 3 hours before bed didn't reduce total sleep time or deep sleep in young adults.
Most public health advice warns against caffeine after 2 PM, based on studies showing sleep disruption. This study contradicts that with objective data from wearables in a real-world setting.
Practical Takeaways
If you drink caffeine before bed and feel like you slept poorly, check your wearable data before assuming it's the caffeine.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 546 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
Human Cohort Study
Subject
Moderate probability
on the GRADE evidence scale
This study watched what happened when people drank coffee or energy drinks before bed and asked them how they slept. It found that even though their watches didn't show they slept less, they felt like they slept worse. But we can't say the caffeine caused that feeling — maybe they were stressed or ate junk food, and that's what really made them feel tired.
No conflicts of interest were detected in this study. No score impact.
Strengths
- Used validated tools (PSQI) and wearable devices for objective and subjective data
- Controlled caffeine dose and timing (240 mg, 3 hours before bed)
- Included a reference period (water) to adjust for fluid intake effects
Weaknesses
- No randomization — participants chose their group (coffee or energy drink)
- No true control group (e.g., placebo or no caffeine)
- Small sample size with no power calculation
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study tried hard to measure sleep with watches and surveys, but people picked their own drinks (coffee or energy drinks), and the group was tiny. That means we can't be sure the results are fair or true for everyone — it's like guessing the taste of pizza after trying one slice from one pizzeria.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
36 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control group+15/15
- Sample size (n=40)+3.6/20
- Follow-up+10/10
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 546 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
This design cannot establish causation — the findings describe an association, not a cause. This is an observational cohort study without randomization or a control group that isolates caffeine as the sole variable. While it compares caffeine intake to a reference period, participants self-selected their group, and many confounders (e.g., stress, diet, mental health) were not measured or controlled, making it impossible to rule out other causes for the observed changes in sleep perception.
No Conflicts
No conflicts of interest identified
No conflicts of interest or funding sources were disclosed in the study text; no industry ties or funder involvement were identified.
The study was conducted at LMU Hospital Munich and approved by its ethics committee, but no funding sources, author affiliations with industry, or conflicts of interest were disclosed. The absence of a COI or funding statement introduces uncertainty, but no evidence of bias or industry influence was found in the text.
Standing
Who’s using this study?
The videos and claims on this site that lean on this study, and the researchers who wrote it.
1 video from Max German cite this study, drawing 1 claim from it.
- Contradicted
Evidence contradicts this claim.
Evidence