GradedHealth

Sleep

Last reviewed October 2026 · Based on 25 studies

Bottom line

Expert panels advise 7 to 9 hours of sleep a night for adults. Large studies link sleeping much less or much more than that to a higher risk of early death, and a steady bedtime and wake time is linked to longer life, but almost all of this evidence is observational, so it shows association rather than cause.

What it helps with

  • Too little sleep raises the risk of early death — Harm, evidence grade B (Moderate). Meta-analyses of prospective cohorts with over 5 million people; observational, so capped at B.
  • Too much sleep raises the risk of early death — Harm, evidence grade B (Moderate). Two meta-analyses of prospective cohorts with over 5 million people; observational, so capped at B.
  • A steady sleep schedule is linked to a longer life — Benefit, evidence grade B (Moderate). Large UK Biobank accelerometer cohort plus a systematic review of cohorts; observational, so capped at B.
  • Irregular sleep is linked to heart and blood sugar problems — Harm, evidence grade C (Limited). Narrative systematic review of 59 mostly observational studies with varied measures and surrogate markers; downgraded for inconsistency and surrogate outcomes.
  • Catch-up sleep on weekends protects the heart and lengthens life — No effect, evidence grade C (Limited). One large UK accelerometer cohort found no link; a 2020 review reported benefit from observational data; conflicting, so downgraded to C.
  • Weekend catch-up lowers the risk of diabetes and dementia — Mixed results, evidence grade C (Limited). Two UK cohorts show lower risk only in subgroups (short weekday sleepers, or 1 to 1.5 hours of catch-up); observational, and not seen in the whole sample of a third cohort.
  • Social jet lag is linked to higher body weight — Harm, evidence grade C (Limited). Meta-analysis of mostly correlational studies with very high heterogeneity and anthropometric surrogates; downgraded for inconsistency and surrogate outcomes.
  • Being an evening type raises the risk of early death — Mixed results, evidence grade C (Limited). Single cohort of 23,854 adults; 9% higher death rate in the adjusted model, mostly attenuated by smoking and alcohol; downgraded for a single study.
  • Too little or too much sleep is linked to memory decline — Harm, evidence grade C (Limited). Umbrella review of 13 systematic reviews; GRADE rates most of the evidence as low certainty.
  • Sleeping more helps short sleepers eat less — Benefit, evidence grade C (Limited). Single 80-person RCT, short and small, measuring energy intake; supported only by a small review of seven studies with preliminary results.
  • A warm bath or shower before bed helps you fall asleep faster — Benefit, evidence grade B (Moderate). One meta-analysis of small trials plus a large observational study and a small randomised trial; downgraded for small trial sizes and reliance on observational data.
  • Progressive muscle relaxation helps you sleep better — Benefit, evidence grade B (Moderate). Two meta-analyses of randomised trials; downgraded one level for very high heterogeneity (I² 86–92%).

What it is

Sleep is the nightly rest period. Recommended durations come from expert panels that reviewed the evidence and voted, not from trials [1]. Sleep timing, meaning when and how regularly you go to bed and wake, is studied as a separate dimension from total hours [2].

How it might work

Proposed mechanisms, not yet tested in people: circadian misalignment (sleep timing out of step with the body clock), autonomic imbalance and systemic inflammation [2].

What the research says

Short and long sleep: in 79 cohort studies, sleeping under 7 hours was linked to a 14% higher death risk and 9 hours or more to a 34% higher risk, compared with 7 to 8 hours [3]. The lowest risk sits near 7 hours [4]. Regularity: in a UK cohort of about 61,000 people, the most regular sleepers had 20% to 48% lower all-cause mortality than the least regular [5]. Irregular timing is also linked to depression, anxiety, insulin resistance, hypertension and cardiovascular events, with moderate certainty in the authors' view [2]. Weekend catch-up sleep: one large UK cohort found no link to death or heart disease [6], while two others found benefits only in some subgroups [7][8]. Social jet lag: linked to higher body mass index in correlational studies [9]. Chronotype: a Finnish cohort found a 9% higher death rate for evening types, mostly explained by smoking and alcohol [10]. Sleep extension: an 80-person trial reduced energy intake by 270 kcal a day [11]. Pre-bed bathing: in 1,094 older adults, bathing 1 to 3 hours before bedtime was linked to falling asleep sooner [12], and a 2019 meta-analysis of 17 studies found that bathing 1 to 2 hours before bed shortened the time to fall asleep [13].

Progressive muscle relaxation. Tensing and then relaxing muscle groups improved sleep quality in pooled randomised trials, with a moderate level of evidence [14][15]. The benefit was not clearly seen in adults aged 55 and older [15]. See the progressive muscle relaxation page.

Doses used in studies

Cohort studies compare short sleep (under 7 hours) and long sleep (9 hours or more) with 7 to 8 hours [3]. The sleep-extension trial enrolled adults who usually slept under 6.5 hours and aimed for 8.5 hours in bed; sleep rose by about 1.2 hours a night [11].

Safety and interactions

Both short and long sleep are linked to higher risk of several conditions [16]. The sleep-extension trial abstract reports no harms [11]. We found no data on pregnancy, breastfeeding or medicine interactions for extending sleep. People with ongoing insomnia, unrefreshing sleep or daytime sleepiness should talk to a doctor.

Caveats and open questions

Most evidence is observational and cannot show that changing sleep changes risk [17]. Older-adult evidence is largely cross-sectional [18]. The cognition evidence is rated low certainty [19]. The authors of the regularity review call for randomised trials of schedule stability [2].

References

  1. National Sleep Foundation's updated sleep duration recommendations: final report.. Sleep Health, 2015.
  2. Sleep regularity as an important component of sleep hygiene: a systematic review.. Sleep Med Rev, 2025.
  3. Imbalanced sleep increases mortality risk by 14-34%: a meta-analysis.. Geroscience, 2025.
  4. Relationship of Sleep Duration With All-Cause Mortality and Cardiovascular Events: A Systematic Review and Dose-Response Meta-Analysis of Prospective Cohort Studies.. J Am Heart Assoc, 2017.
  5. Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study.. Sleep, 2024.
  6. Device-measured weekend catch-up sleep, mortality, and cardiovascular disease incidence in adults.. Sleep, 2024.
  7. Associations of accelerometer-measured weekend sleep catch-up and loss with risk of type 2 diabetes: a prospective cohort study.. Diabetes Res Clin Pract, 2025.
  8. Accelerometer-measured weekend catch-up sleep and incident dementia: A prospective cohort study.. Alzheimers Dement, 2025.
  9. Social jetlag and obesity: A systematic review and meta-analysis.. Obes Rev, 2024.
  10. Chronotype and mortality - a 37-year follow-up study in Finnish adults.. Chronobiol Int, 2023.
  11. Effect of Sleep Extension on Objectively Assessed Energy Intake Among Adults With Overweight in Real-life Settings: A Randomized Clinical Trial.. JAMA Intern Med, 2022.
  12. Hot-water bathing before bedtime and shorter sleep onset latency are accompanied by a higher distal-proximal skin temperature gradient in older adults.. J Clin Sleep Med, 2021.
  13. Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis.. Sleep Med Rev, 2019.
  14. Progressive muscle relaxation technique improves sleep quality and mental health: A systematic review and meta-analysis of randomized controlled trials.. J Psychosom Res, 2026.
  15. The effects of progressive muscle relaxation on sleep quality in adults: a systematic review and meta-analysis of randomized controlled trials.. Front Public Health, 2026.
  16. Sleep duration and health outcomes: an umbrella review.. Sleep Breath, 2022.
  17. Long sleep duration and health outcomes: A systematic review, meta-analysis and meta-regression.. Sleep Med Rev, 2018.
  18. Sleep quality and duration and frailty in older adults: a systematic review.. Front Public Health, 2025.
  19. Duration and Quality of Sleep as Risk Factors for Cognitive Impairment: An Umbrella Review of Systematic Reviews.. Int J Geriatr Psychiatry, 2026.