Sleep

Sleep is the highest-leverage longevity lever most adults take for granted. Cohort data covering tens of millions of person-years consistently show that short sleep (under about six hours), long sleep (over about nine), irregular timing, and untreated sleep apnea each independently predict all-cause mortality, cardiovascular disease (CVD), dementia, type 2 diabetes, and depression. The American Heart Association formally added sleep to its core cardiovascular-health metric in 2022, putting it on the same line as blood pressure and lipids.^[1] A UK Biobank analysis of 385,292 adults found that those with a healthy sleep pattern — early chronotype, 7–8 hours, no insomnia symptoms, no snoring, no daytime sleepiness — had roughly 35% lower CVD risk than those at the opposite end.^[2]

Despite that centrality, sleep is also one of the most-meddled-with and least-understood health behaviours. The 8-hour rule is approximately right; regularity matters at least as much as duration; and most over-the-counter sleep aids are weak or actively harmful at scale.

What the evidence actually supports

Strong:

• 7–8 hours of sleep on a regular schedule is associated with the lowest cardiovascular and all-cause mortality risk. The dose-response is U-shaped: short sleep raises mortality by about 14% and long sleep by about 34% versus the 7–8 hour reference.^[3] The long-sleep arm partly reflects underlying illness (depression, untreated apnea, frailty), but the short-sleep arm has held up under genetic-instrument analyses that limit reverse causation.
• Sleep regularity — going to bed and waking up at consistent times — independently predicts mortality at least as strongly as duration. A 2023 UK Biobank wrist-accelerometer study of almost 89,000 adults found 49% lower all-cause mortality in the most-regular fifth of sleepers compared with the least-regular.^[4] Irregular timing also raised major adverse cardiovascular events by 26% — and adequate total hours did not offset it.^[5] See Circadian rhythms for why timing turns out to matter as much as duration.
• Light at night. Wrist-light-sensor data from ~89,000 UK Biobank adults found the brightest-night group had 21–34% higher all-cause mortality, and a parallel analysis found a 53% higher risk of incident type 2 diabetes (HR 1.53) — dose-dependent and independent of genetic risk.^[6] Keep the bedroom dark; the cheapest longevity lever in this whole list may be turning the lights off.
• Cognitive Behavioural Therapy for Insomnia (CBT-I) is the first-line treatment for chronic insomnia. It outperforms hypnotic drugs in head-to-head trials and effects persist after treatment ends.^[7] See Treating chronic insomnia for the case against chronic Z-drugs and benzodiazepines.
• Untreated obstructive sleep apnea (OSA) substantially raises mortality, hypertension, atrial fibrillation, stroke, and dementia risk. Continuous positive airway pressure (CPAP) reliably improves symptoms, daytime sleepiness, blood pressure, and atrial-fibrillation recurrence; whether it cuts hard cardiovascular events is a genuine live controversy — the major intent-to-treat RCTs were null (in largely non-sleepy patients with low adherence), while adherence-adjusted and observational analyses suggest benefit above the four-hour mark. OSA is dramatically underdiagnosed in midlife — globally, an estimated 936 million adults aged 30–69 have it.^[8]30198-5/abstract) See Sleep-disordered breathing.

Moderate:

• Light exposure — bright morning light plus dim, warm evening light — strengthens circadian alignment and improves both sleep and mood.
• Cool bedroom (18–20 °C / 65–68 °F) supports the natural core-temperature drop that initiates sleep.
• Magnesium glycinate (200–400 mg) has small but real effects on subjective sleep, particularly in adults with low dietary intake.^[9] See Sleep supplements for the rest of the supplement landscape — melatonin, L-theanine, ashwagandha, glycine, valerian, CBD — and which (if any) are worth using long-term.
• A 20-minute power nap is one of the most efficient cognitive boosters in physiology. A daily 90-minute nap in a well-rested midlife adult is a different signal — it tracks with cardiovascular disease, dementia, and earlier death, though much of that signal is the nap acting as a marker of underlying illness rather than its cause. See Daytime naps for when the harm signal softens — and when a long nap is genuinely fine.

Weak / preliminary:

• L-theanine, ashwagandha, glycine, and low-dose melatonin all have modest, narrow indications. None are first-line for chronic insomnia.

Caution:

• Chronic use of benzodiazepines, Z-drugs, and over-the-counter antihistamines (Benadryl, ZzzQuil) — particularly above age 50 — increases falls, fractures, cognitive impairment, and dementia risk.^[10]
• Alcohol as a sleep aid fragments sleep architecture — it suppresses rapid-eye-movement (REM) sleep and increases wakefulness in the second half of the night, even at low doses.
• Night-shift work is classified by the World Health Organization's cancer agency as probably carcinogenic to humans (Group 2A), based on consistent breast- and prostate-cancer signals.^[11]

Why sleep is a longevity lever

Sleep is not passive downtime. It's the window in which the brain clears its metabolic waste, the body releases the bulk of its growth hormone, autonomic tone resets, and several of the molecular machines that track biological age are at their most active.

• Glymphatic clearance. The brain's perivascular drainage network — the glymphatic system — clears interstitial waste including amyloid-β and tau during slow-wave (deep) sleep. The first direct human evidence of this came from a 2026 randomised crossover study showing that one night of normal sleep raised morning plasma amyloid-β and tau biomarkers compared with sleep deprivation, in proportion to electroencephalographic (EEG) slow-wave activity.^[12] A Framingham follow-up found each percentage-point yearly decline in slow-wave sleep was associated with about 27% higher dementia risk over 17 years.^[13]
• Hormones. Roughly 70% of daily growth-hormone output in men occurs around sleep onset; cortisol reaches its nadir during early-night deep sleep; melatonin amplitude falls steeply with age (an 80-year-old produces only about 10% of teenage levels). The endocrine cost of bad sleep is real and measurable: one week of 5 h/night drops daytime testosterone by 10–15% in healthy young men — equivalent to roughly a decade of normal aging.
• Epigenetic aging. Short sleep and insomnia accelerate second-generation epigenetic clocks. In the Health and Retirement Study, insomnia carried about 0.7 years of GrimAge acceleration and short sleep about 1.5 years; combined, the effect compounded.^[14] Crucially, the signal is modifiable — treating insomnia in older adults reduces senescence-associated gene expression and slows the DunedinPACE rate-of-aging clock.

The mechanistic upshot: bad sleep doesn't just feel bad. It accelerates the cellular processes that produce the diseases that fill the last decade of an average adult's life.

Sleep regularity matters as much as duration

The most consequential recent shift in sleep epidemiology is the recognition that when you sleep matters at least as much as how long. The Sleep Regularity Index (SRI), measured by wrist accelerometry, captures the probability of being in the same sleep/wake state at any two points 24 hours apart. In the UK Biobank (88,975 adults), the most-regular fifth had 49% lower all-cause mortality, 57% lower cardiometabolic mortality, and 39% lower cancer mortality than the least-regular — and regularity outperformed duration as a mortality predictor in head-to-head models.^[15] A 2025 state-of-the-art review concluded that sleep irregularity is independently a robust risk factor for cardiometabolic disease.^[16] A separate UK Biobank dementia analysis found irregular sleep timing predicted incident dementia over a decade of follow-up.^[17]

Mechanistically, the master clock in the hypothalamus coordinates roughly 20,000 peripheral cellular clocks gating DNA repair, autophagy, nutrient sensing, and the immune response. The system runs on a slightly-longer-than-24-hour period (~24.2 hours) and needs daily re-entrainment — by light, meal timing, and routine — to stay aligned.^[18] When central and peripheral clocks fall out of phase, the consequences spread across metabolism, immunity, and cardiovascular tone. Catching up on weekends partially restores subjective alertness but does not reverse the weekday metabolic cost.

The practical upshot is short: anchor your wake time first, get morning light, dim the evening, and eat earlier. Circadian rhythms covers the full picture — light dosing, chrononutrition, the BMAL1–mTOR axis, jet lag, and shift work — and the surprisingly small set of habits that move all of it at once.

Sleep-disordered breathing: the underdiagnosed cardiovascular risk

OSA is the repeated partial or complete collapse of the upper airway during sleep. It affects an estimated 30–40% of middle-aged men and 15–20% of middle-aged women, and the majority of cases are undiagnosed. The mortality and morbidity signals are large: a 27-cohort meta-analysis covering more than 3 million adults found severe OSA roughly doubled all-cause mortality and nearly tripled cardiovascular mortality.^[19] Untreated moderate-to-severe OSA roughly doubles stroke and death risk over six years.^[20] The damaging mechanism isn't the airway collapse itself but the intermittent-hypoxia pattern of hundreds of nightly oxygen drops and re-oxygenations — an ischemia-reperfusion-like injury that drives reactive oxygen species, chronic low-grade inflammation, endothelial dysfunction, and accelerated telomere attrition.

Screening is cheap and the diagnostic pathway has shortened. The validated STOP-Bang questionnaire — eight yes/no questions on snoring, tiredness, observed apneas, hypertension, BMI, age, neck size, and sex — flags high-risk adults; three or more "yes" answers warrants a sleep study.^[21] Home sleep apnea testing is now first-line for most adults; full in-lab polysomnography is reserved for complex cases. Treatment works when actually used: CPAP at four or more hours per night on at least 70% of nights reduces blood pressure, atrial-fibrillation recurrence, and motor-vehicle accidents.^[22] The headline "negative" SAVE trial averaged only 3.3 hours of nightly CPAP use — adherence-adjusted analyses consistently show cardiovascular benefit above the four-hour mark. For obesity-driven OSA, a 10% body-weight reduction cuts the apnea-hypopnea index by roughly 25%, and GLP-1 receptor agonists are now a documented option: tirzepatide significantly reduced apnea severity in adults with moderate-to-severe OSA in SURMOUNT-OSA.^[23]

Sleep-disordered breathing covers the full diagnostic pathway, the alternatives when CPAP isn't tolerated (mandibular advancement devices, positional therapy, hypoglossal-nerve stimulation, surgery), the honest assessment of mouth taping, and orofacial myofunctional therapy.

Chronic insomnia: behaviour outperforms pills

Roughly 10–15% of adults have chronic insomnia (difficulty falling or staying asleep, three or more nights a week, for at least three months). The treatment landscape flipped in 2017: CBT-I is now first-line, ahead of any drug. CBT-I outperforms hypnotic medications in head-to-head trials and the effects persist after treatment ends — unlike drugs, which lose effect when discontinued.^[24] The five components — stimulus control, sleep restriction (the most powerful single element), cognitive restructuring, sleep hygiene, and relaxation — are now delivered at scale by validated digital programmes that have randomised-trial support comparable to in-person therapy.

When drugs are genuinely needed (as a bridge, for short-term acute stress, or after CBT-I has failed), the safety hierarchy has shifted. Newer dual orexin-receptor antagonists (suvorexant, lemborexant, daridorexant) cut sleep-onset latency, raise total sleep time, and lower Insomnia Severity Index scores without producing dependence, tolerance, or rebound insomnia in a 2025 network meta-analysis of 5,198 participants.^[25] Z-drugs (zolpidem and relatives) are reserved for short courses of four weeks or less; benzodiazepines and over-the-counter antihistamines like diphenhydramine carry documented falls-and-dementia signals and sit on the Beers Criteria for avoidance in older adults.^[26] A 2025 simulation in Sleep estimated substantial cumulative falls and cognitive-impairment risk from chronic prescription sleep medications in adults over 50.^[27]

Treating chronic insomnia covers the full CBT-I protocol, the pharmacotherapy tier system in detail, and the special populations — menopausal vasomotor insomnia, men with low energy and libido who should be screened for OSA before pursuing testosterone, and older adults for whom the prescribing margin is narrow.

Daytime naps: opposite ends of the longevity ledger

The same behaviour at different durations sits on opposite ends of the ledger. A 20-minute power nap measurably restores vigilance, working memory, mood, and athletic readiness — and an MIT workplace trial of a daily nap opportunity raised data-entry productivity by 2.3%. A 90-minute habitual nap in a sedentary middle-aged adult points the other way: a 2026 umbrella review of 16 meta-analyses covering 244 outcomes found naps over 60 minutes associated with roughly 30% higher coronary-heart-disease risk and elevated diabetes, obesity, and all-cause mortality risk.^[28] A Mendelian-randomisation study using genetic predictors of napping behaviour supports a causal link to coronary artery disease (odds ratio about 1.47).^[29] Objective actigraphy data from the Rush Memory and Aging Project found each additional hour of daily napping was associated with about 13% higher mortality.^[30]

But the harm signal is conditional, not universal. It concentrates in people who are already sleeping enough at night — in short sleepers a midday nap can offset the cardiovascular risk of a bad night, and a habitual short (~30-minute) siesta looks protective in several Mediterranean cohorts. The most useful contemporary framing: in older adults, excessive daytime napping is more likely a biomarker of decline than its cause. Habitual long nappers consistently show elevated C-reactive protein and interleukin-6 — inflammation makes you tired, and the nap is the body responding to a problem upstream (commonly undiagnosed OSA, cardiometabolic disease, or early neurodegeneration). Activity context also matters: an athlete recovering from a hard training session benefits from a longer nap; a sedentary worker stacking 90 minutes of immobility on top of an already-still day does not.

Daytime naps covers the dose-response in detail, Process O and the sleep-switch, the cardio-metabolic and mortality data, athletic vs. sedentary context, and when "creeping nap length" is a reason to investigate further rather than indulge.

Sleep supplements: modest at best

Most "natural" sleep aids deliver small effects with few side effects, but only a couple have real evidence and several popular ones are oversold. The honest summary across magnesium, L-theanine, melatonin, ashwagandha, glycine, valerian, and CBD: none replace CBT-I for chronic insomnia and none produce clinically meaningful tolerance, dependence, or withdrawal — though "no withdrawal" is not the same as "harmless for years," a distinction that now matters most for melatonin.

The two with the best risk-benefit profile are magnesium glycinate (200–400 mg, moderate evidence with stronger effects in adults with low dietary intake) and L-theanine (200 mg, weak-to-moderate evidence, excellent safety). Low-dose melatonin (0.3–1 mg) earns its place for narrow indications — jet lag, delayed sleep-phase syndrome, insomnia in older adults whose endogenous melatonin has fallen — but the AASM 2017 guideline gave a conditional recommendation against it for general insomnia, and over-the-counter products in the US vary from 83% to 478% of labelled dose.^[31] Higher doses (3–10 mg) are not more effective; they often produce next-day grogginess. And the "melatonin is harmless" framing has softened: a preliminary 2025 analysis linked long-term daily use to higher heart-failure and mortality rates — confounded and not yet peer-reviewed, but enough reason to keep melatonin to low doses and narrow circadian indications rather than an indefinite nightly habit.^[32] Ashwagandha, glycine, valerian, and CBD all sit at the weak-to-preliminary end, with ashwagandha carrying rare hepatotoxicity reports and CBD producing meaningful drug interactions via the cytochrome-P450 system.

Sleep supplements covers the dosing, mechanisms, safety profile, and tier ranking for each, plus the honest answer to the addiction question for L-theanine and magnesium.

Practical sleep checklist (evidence-weighted)

1. Anchor your wake time first. Same time every day, weekends included. The central nervous system adapts to consistent waking; it does not adapt to consistent sleep onset.
2. Get morning light (10–30 minutes outdoors within an hour of waking). Outdoor midday light delivers 50,000–100,000 lux versus 500–1,000 indoors — the order-of-magnitude gap is the point.
3. Dim the evening. Reduce overhead lighting and screens 1–2 hours before bed. Even moderate room light (~100 lux) can suppress melatonin by half in sensitive individuals.
4. Cool, dark bedroom. 18–20 °C (65–68 °F); blackout curtains; remove glowing electronics. A warm bath or shower (40–42.5 °C) about 90 minutes before bed exploits the same mechanism in reverse — skin vasodilation speeds the core-temperature drop and shortens sleep-onset latency.^[33]
5. No caffeine within 10–12 hours of bed. Half-life is 3–6 hours but varies widely; the closer to bed, the larger the architectural cost.
6. No alcohol within three hours of bed. It hastens sleep onset but fragments architecture and suppresses REM at low doses.
7. Eat earlier. Last meal 2–3 hours before bed. Late, large meals delay peripheral clocks and degrade sleep architecture.
8. Screen yourself for OSA if you snore loudly, have witnessed apneas, hypertension, atrial fibrillation, or a body mass index of 30 or above. Take the ^[34]; three or more "yes" answers warrants a sleep study.
9. For chronic insomnia, start with CBT-I, not pills. Validated digital programmes have trial support comparable to in-person therapy.
10. Cap daytime naps at 20–30 minutes and place them in the early-to-mid afternoon. If you regularly need more, investigate the cause rather than indulge it.
11. Don't mouth-tape without a clinical evaluation. Treat nasal obstruction first.

What's overrated

• Catching up on weekends. Partial subjective recovery; the weekday metabolic and cognitive deficits don't reverse.
• Blue-light-blocking glasses worn all evening as a sole intervention. Modest evidence; dimming the room matters more.
• High-dose evening melatonin (3–10 mg). Often exceeds the dose that maximises sleep onset and produces next-day grogginess. A 2025 cohort also linked long-term use to higher heart-failure and mortality rates — observational and confounded, but a reason to keep melatonin to low doses and narrow circadian indications rather than an indefinite nightly habit.^[35]
• Mouth taping as a viral biohack. Defensible only after an ear-nose-throat (ENT) evaluation has cleared nasal obstruction; otherwise risk-asymmetric.
• Snoring apps that "diagnose" OSA. Acoustic snoring metrics correlate poorly with the apnea-hypopnea index. Use them to motivate a sleep study, not as a substitute.
• Alcohol as a sleep aid. It hastens onset but suppresses REM and fragments the second half of the night.
• "Don't exercise in the evening." Mostly a myth: meta-analysis shows evening exercise does not disrupt sleep overall, with only acute high-intensity sessions ending within a few hours of bed slightly trimming REM.^[36] Vigorous exercise within about an hour of bed may still delay onset in sensitive people.