How much sleep does an individual "need" to be healthy? The answer is, we don't yet know! The current belief by sleep doctors is that an individual who meets all of the following criteria is likely getting "enough" sleep:
(i) sleeps the same amount on work days and non-work days,
(ii) awakens without an alarm clock,
(iii) does not use caffeine or other stimulants to remain awake or substances to fall asleep,
(iv) does not fall asleep within five minutes or in other non-stimulating conditions (e.g., when a passenger in a car)
(v) is healthy.
If even one of those conditions is not met, then the individual is probably not obtaining sufficient sleep. The answer, though, may not be as clear when we look at different populations.
In the industrial world, short sleep duration -- the amount of time one is asleep -- has been linked with multiple adverse health outcomes, including type II diabetes, weight gain, mood disorders, learning disabilities, decreased immune function to fight off illness, impaired performance and alertness, motor vehicle accidents and industrial accidents.
Two recent published reports on sleep duration in "hunter-gatherer" societies, or other communities living without electricity, have documented shorter sleep duration than that recommended by sleep experts in the industrial world. It is assumed, but not proven, that the health of the people studied is not negatively affected by this shorter sleep. Other reports have found longer sleep durations in societies without electricity, which align with recommendations by sleep experts.
The type of society that an individual lives in is only one factor that affects the amount of sleep an individual gets. Some (but not all) factors include:
- The "drive" for sleep, which increases the longer you are awake and decreases during sleep;
- The body's internal sleep/wake clock, which biologically promotes both wake and sleep dependent on the time of day;
- Environmental conditions such as room temperature.
In industrial societies, sleep is often restricted by choice (e.g., to be with others, watch TV, use the internet) or for work-related reasons (e.g., if you must work multiple jobs or if your job requires long hours).
Sleep research that we have conducted at Brigham and Women's Hospital in Boston has shown that healthy individuals sleep for far shorter periods of time when they are living at home than when they are in a controlled setting where they must remain in bed for longer than they usually do at home.
"If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made,'' wrote Allan Rechtschaffen (a leading sleep researcher).
During sleep, creatures are not eating, procreating or capable of defending themselves and yet its presence is ubiquitous (existing in organism as diverse as flies and humans), obligatory (animals die if deprived of sleep), and complex (comprising both NREM and REM sleep stages).
And while Rechtschaffen's quote makes a lot of sense, we still don't know what the function(s) of sleep are, although we can infer it based on what we know about the dangers of insufficient sleep. It could be possible, for example, to obtain enough sleep to meet immune function "needs" but not enough for metabolic "needs." Therefore, it is possible that you might feel "rested" after sleeping six hours, but your metabolic recovery is not yet complete. The science to answer these questions has not yet been done.
Unlike treatments for many medical conditions, sleep does not require out-of-pocket or insurance payments, does not require training or special equipment, does not require doctor/nurse appointments or prescriptions, can be done at home or elsewhere and therefore has no travel requirements, has no side effects and has multiple health (including mental health) benefits.
Health care professionals, therefore, strongly encourage individuals to sleep more. If you are tired after being in bed for 8 hours and/or your bed-partner says you snore or stop breathing during sleep and/or you have questions about the quality or quantity of your sleep, please consult a health professional.
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