How to sleep better

The best-measured factor on this page. A meta-analysis of 24 controlled studies found caffeine cut total sleep time by 45 minutes and sleep efficiency by 7%, lengthened sleep onset by 9 minutes, added 12 minutes of waking after sleep onset, and swapped about 11 minutes of deep sleep for light sleep [1]. The same analysis modeled clearance: a 107 mg coffee should land at least 8.8 hours before bed, and a 217 mg pre-workout dose about 13.2 hours before. A single 400 mg dose taken 6 hours before bed cut objectively measured sleep by about an hour, and people did not report noticing [8].

No habit fixes a night that is being broken by breathing. Sleep apnea fragments sleep dozens of times an hour without waking you fully, and in the Wisconsin Sleep Cohort untreated severe apnea carried about 3x all-cause mortality over 18 years [9]. It is common, underdiagnosed, and usually announces itself as loud snoring, gasping, or exhaustion despite enough hours in bed.

Sleep regularity, how similar each night is to the last, tracks with the outcomes that matter: in 60,977 wearable-tracked UK Biobank adults, the most regular sleepers had 20 to 48% lower all-cause mortality and regularity predicted lifespan better than duration [10]. A stable schedule also lets the circadian clock time melatonin and body temperature correctly, which is what makes falling asleep easy rather than a fight.

Light is the master input to the circadian clock. A week of camping with only natural light shifted melatonin onset about 2 hours earlier and lined internal night up with actual darkness [3]. In the other direction, reading on a light-emitting tablet for 4 hours before bed suppressed melatonin, delayed the circadian clock by about 1.5 hours after 5 nights, lengthened time to fall asleep, reduced REM sleep, and lowered next-morning alertness compared with a printed book [4].

A review of 27 studies found alcohol shortened time to fall asleep and deepened the first half of the night, then suppressed REM and fragmented the second half, with the disruption growing with dose [5]. The nightcap is the most common self-inflicted sleep disorder because it feels like it works.

Core temperature has to fall for sleep to start and deepen. In 50 older adults monitored at home across nearly 11,000 person-nights, sleep efficiency and restfulness peaked with bedroom air between 20 and 25°C and dropped by 5 to 10% as it rose from 25 to 30°C, with real differences between individuals in their best setting [6].

A meta-analysis of 66 studies found regular exercise had small benefits for total sleep time and sleep efficiency, small-to-medium for time to fall asleep, and moderate benefits for self-rated sleep quality; even a single session helped slightly on the same night [11]. In a 1,591-person Fitbit cohort, activity features explained more of the variance in measured sleep than any other lifestyle or blood measure (Cavon et al. [12], preprint).

In students tracked through exam season, perceived stress explained more of the variation in sleep quality than caffeine or alcohol, and once stress was in the model caffeine stopped being significant [13]. When poor sleep settles into 3 or more nights a week for 3 months, it is insomnia, and cognitive behavioral therapy for insomnia is the first-line treatment in the American College of Physicians guideline, matching or beating medication with benefits that persist after treatment ends [7].

A meta-analysis of 11 cohorts found naps of an hour or more associated with higher cardiovascular disease and all-cause mortality, while naps under 30 minutes carried no such signal [14]. Long or late naps also spend the sleep pressure that a good night needs.

In 1,218 male twins aged 51 to 60, additive genetic effects explained 34% of the variance in global Pittsburgh Sleep Quality Index scores and 31% of the odds of being a poor sleeper; shared family environment explained none of either, and the remaining two-thirds was each twin’s own environment [2]. Component heritabilities ran 23 to 34%, and sleep-medication use was not heritable at all.