Sleep

Sleep is a recurring state of reduced awareness and responsiveness to the surroundings. Its timing is controlled by two systems: a sleep pressure that builds during waking hours and a roughly 24-hour internal clock that is reset each day mainly by light [1, 2]. Each night consists of repeating cycles of non-REM and REM (rapid eye movement) sleep, each lasting about 90 to 120 minutes after the first [3]. These systems and stages are described in sleep physiology.

Sleep duration is consistently associated with lifespan. In a meta-analysis (a study that pools the results of earlier studies) of 1,382,999 people, habitual short sleep was associated with 12% higher mortality (risk of death) and long sleep with 30% higher mortality than sleep of about 7 hours [4]. Short sleep in midlife has also been linked to a 30% higher risk of dementia [5], and adults with five healthy sleep habits had an estimated 4.7 (men) and 2.4 (women) more years of life expectancy at age 30 than those with one or none [6]. These findings come from observational studies, which track people over time without assigning a sleep pattern, and part of the long-sleep association is thought to reflect existing illness.

The most common sleep disorder is insomnia, persistent difficulty falling or staying asleep with daytime impairment, which affects up to 10% of adults in its chronic form [7]. Cognitive behavioral therapy is the recommended first treatment [8]. Obstructive sleep apnea, a separate breathing disorder, affects an estimated 936 million adults aged 30 to 69 worldwide [9]. Compounds studied for sleep, including melatonin, magnesium and glycine, are covered under sleep supplements.

The section also covers sleep duration and mortality, sleep quality and its measurement, and daytime naps, each drawing on the YapSpan deep dives on the same subjects.