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Sleep: what is it, why does it matter, and how much do we need?

Yesterday I listened to Professor Colin Espie lecture on "What is sleep ... and why does it matter?".   I then went on to his packed two hour workshop on "Assessing and treating insomnia in everyday clinical practice".  I've heard Colin lecture before and been to a full day workshop with him as well, but it's great to get an update on where sleep research & treatment has got to.  Colin is a professor of Sleep Medicine linked with the Oxford 'Sleep & Circadian Neuroscience Institute' and is very much a world expert in this area. 

He states "There is nothing that is more fundamental to behavioural, cognitive and emotional functioning than sleep. Like breathing, sleep is a largely involuntary behaviour that is essential to life, and it occurs unfailingly as part of an approximate 24-hour cycle across the lifespan".   Well, all-night parties or convulsive work binges aside, that makes good sense.  Sleep and wakefulness are interdependent with the sleep researchers Cirelli & Tononi, in their 2008 paper "Is sleep essential?", commenting "Sleep is the price we pay for wakefulness".  They argue that "Sleep is universal (across animal species), tightly regulated, and cannot be eliminated without deleterious consequences". Their 2017 paper "The sleeping brainbrings us more up to date, arguing that sleep is crucial for "brain plasticity" ... that there is essential house-keeping work occurring in the brain while we sleep without which we develop increasing cognitive impairments ... as well as suffering damaging effects on metabolism, immunity & other functions.

So how much sleep do we need?  I asked Colin this and he surprised me by saying that asking "How much sleep do we need?" is a bit like asking "How tall should we be?".  Mm ... I think he memorably makes a good point here.  It's easy to make sweeping statements claiming we should all be getting 7 to 8 hours sleep each night, but this reasonably accurate comment hides considerable underlying variation in sleep needs.  There is a scientific narrative that suggests we're facing some kind of sleep crisis, so the 2015 consensus statement of the American Academy of Sleep Medicine baldly states that "Adults should sleep 7 or more hours per night on a regular basis to promote optimal health.  Sleeping less than 7 hours per night on a regular basis is associated with adverse health outcomes." and it is only further into the article that the authors concede that this is, in fact, a general recommendation where "Individual variability in sleep need is influenced by genetic, behavioral, medical, and environmental factors."   I think the 2015 US National Sleep Foundation's paper presents the data with more appropriate caution.  They give a chart highlighting "recommended", "may be appropriate", and "not recommended" durations of sleep across the lifespan.  The Australian Sleep Health Foundation clarify this chart by providing - in their long list of helpful freely downloadable sleep health fact sheets - a two page handout titled "How much sleep do you need?" which lists the NSF recommendations in table form. 

Sensibly they precede the table with the words "Different people have different sleep needs. The advice in the table below is only a guide. You can make a good guess if a person is sleeping enough at night - observe how they act and function during the day."   

sleep needs
age recommended may be appropriate not recommended

newborns, 0-3 months


14-17 hours 11-13 hours, 18 to 19 hours less than 11 hours, more than 19 hours
infants, 4-11 months 12-15 hours 10-11 hours, 16 to 18 hours less than 10 hours, more than 18 hours
toddlers, 1-2 years 11-14 hours 9-10 hours, 15-16 hours less than 9 hours, more than 16 hours
preschoolers, 3-5 years 10-13 hours 8-9 hours, 14 hours less than 8 hours, more than 14 hours
school-aged children, 6-13 years 9-11 hours 7-8 hours, 12 hours less than 7 hours, more than 12 hours
teenagers, 14-17 years 8-10 years 7 hours, 11 hours less than 7 hours, more than 11 hours
young adults, 18-25 years 7-9 hours 6 hours, 10-11 hours less than 6 hours, more than 11 hours
adults, 26-64 years 7-9 hours 6 hours, 10 hours less than 6 hours, more than 10 hours
older adults, >64 years 7-8 hours 5-6 hours, 9 hours less than 5 hours, more than 9 hours

As children age they need less sleep, with more standard adult requirements emerging by around age 20. Teenagers typically want to go to bed later & sleep in.  Older people may well spend more time in bed, but their sleep needs are actually fairly similar to younger adults.  For more on this there are numerous easily available online resources - for example the many Australian Sleep Health Foundation's handouts are excellent, as too is the information provided by the American Academy of Sleep Medicine's Sleep Education website.

However, as the American Academy of Sleep Medicine highlighted "Individual variability in sleep need is influenced by genetic, behavioral, medical, and environmental factors."   So as with the "How tall should we be?" question, there is considerable genetic variation.  Gasperi & colleagues studied 400 twins and found a genetic heritability of about 37% for sleep quality (this study was on quality rather than duration).  And a still larger study on twins by Watson estimated that 39% of sleep duration is heritable.


The widely available WSAS scale can help here ... and a simple home test can be useful too ... Kitamura, S., et al. (2016). "Estimating individual optimal sleep duration and potential sleep debt." Scientific Reports 6: 35812.



 At the same time, 

Oscar Wilde wrote "The truth is rarely pure and never simple".  He certainly had a point with the claims and counter-claims about sleep needs.  So there seems considerable truth to statements that too little or too much sleep is bad for us, and that a large number of people struggle with sleep.  However, on average we're probably less sleep-deprived in modern society than alarmists claim, reductions in average sleep times are probably not becoming much worse over time, and many sleep complaints may be exaggerated.  Let's look at these points in a little more detail. 


Australian Sleep Health Foundation "How much sleep do you need?"  Freely downloadable fact sheet.

Carskadon, M. A., et al. (1976). "Self-reports versus sleep laboratory findings in 122 drug-free subjects with complaints of chronic insomnia."  American Journal of Psychiatry 133(12): 1382-1388.  

Cirelli, C. and G. Tononi (2017). "The sleeping brain."  Cerebrum: Dana Forum on Brain Science 2017: cer-07-17. 

Cirelli, C. and G. Tononi (2008). "Is sleep essential?"  PLoS Biology 6(8): e216.

Gasperi, M., et al. (2017). "Genetic and environmental influences on sleep, pain, and depression symptoms in a community sample of twins."  Psychosomatic Medicine 79(6): 646-654. 

Hirshkowitz, M., et al. (2015). "National Sleep Foundation’s updated sleep duration recommendations: final report." Sleep Health: Journal of the National Sleep Foundation 1(4): 233-243.

Kitamura, S., et al. (2016). "Estimating individual optimal sleep duration and potential sleep debt." Scientific Reports 6: 35812.

Manconi, M., et al. (2010). "Measuring the error in sleep estimation in normal subjects and in patients with insomnia."  J Sleep Res 19(3): 478-486.

Watson, N. F., et al. (2015). "Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society." Sleep 38(6): 843-844.

Watson, N. F., et al. (2016). "Sleep duration and area-level deprivation in twins."  Sleep 39(1): 67-77.

More to follow ...

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