Adult
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual’s circadian rhythms. A person’s major sleep episode is relatively inefficient and inadequate when it occurs at the “wrong” time of day. The timing is correct when the following two circadian markers occur after the middle of the sleep episode but before awakening:
- maximum concentration of the hormone melatonin, and
- minimum core body temperature.
The National Sleep Foundation in the United States maintains that seven to nine hours of sleep for adult humans is optimal and that sufficient sleep benefits alertness, memory and problem solving, and overall health, as well as reducing the risk of accidents. A widely publicized 2003 study performed at the University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with fewer than eight hours of sleep.
However, a University of California, San Diego psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night. Another study of sleep duration and mortality risk in women showed similar results. Other studies show that “sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality”, though this study suggests the cause is probably other factors such as depression and socio-economic status which would correlate statistically. It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.
Researchers from the University of Warwick and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but that too much sleep can also double the risk of death. Professor Francesco Cappuccio said: “Short sleep has been shown to be a risk factor for weight gain, hypertension and Type 2 diabetes sometimes leading to mortality but in contrast to the short sleep-mortality association it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status and cancer-related fatigue. In terms of prevention, our findings indicate that consistently sleeping around 7 hours per night is optimal for health and a sustained reduction may predispose to ill-health.”
Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism and bipolar disorder. Up to 90% of patients with depression are found to have sleep difficulties.
Hours by age
Children need a greater amount of sleep per day than adults to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost 9 hours a day in REM-sleep. By the age of five or so, only a bit over two hours are spent in REM.
| Age and condition | Average amount of sleep per day |
|---|---|
| Newborn | up to 18 hours |
| 1–12 months | 14–18 hours |
| 1–3 years | 12–15 hours |
| 3–5 years | 11–13 hours |
| 5–12 years | 9–11 hours |
| Adolescents | 9-10 |
| Adults, including elderly | 7–8 (+) hours |
| Pregnant women | 8 (+) hours |
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