Some of the world's greatest leaders, from Napoleon Bonaparte and Winston Churchill to Bill Clinton and Margaret Thatcher, are said to have managed on just four to six hours' sleep a night, whereas the typical teenager finds it difficult to get out of bed in less than 10.
Newborn babies can sleep for up to 18 hours – admittedly at irregular intervals – whereas an elderly person may find it hard to sleep longer than six, although they often have to resort to the odd afternoon nap to make up for what they lacked at night.
Sleep is the quintessential ingredient of life. Every animal does it at some point in the 24-hour cycle and people who are forcibly deprived of sleep are effectively undergoing torture. But the big unanswered question is how much sleep do we actually need?
Some people seem happy with four or five hours, although most people would feel sleep-deprived on less than six. Others need a good seven or eight hours of sleep and adolescents are renowned for extended kips.
So how much sleep is necessary for a healthy mind and body, and does this amount truly need to vary between people and age groups?
The latest study into sleep may help to resolve the issue with the discovery that certain people in the population carry the smallest of genetic mutations in a gene that appears to play a significant role in deciding just how much sleep human beings need.
Scientists studied an extended family in California and found that a mother and her daughter shared a life-long habit of rising in the very early hours of the morning with no apparent ill-effects. They routinely went to bed between 10.30pm and 11pm and got up between 4am and 4.30am.
The researchers took blood samples from all members of the family and analysed their DNA for any signs that could explain this unusual behaviour. The tests revealed that the mother and her daughter did in fact share a tiny "point mutation" in a gene known as hDEC2, which is known to affect the regulation of other genes and has been implicated in the control of sleeping patterns in animals.
Other members of the family who followed a more conventional sleeping pattern were not found to have inherited the same mutation. These family members typically required the normal eight hours or so of sleep a night instead of the five to six hours of the mother and daughter.
Just to make sure that the hDEC2 mutation was truly involved in this unusual sleeping pattern, rather than a coincidental occurrence in the two women, the scientists went on to create genetically-engineered mice with the same point mutation to the same gene. These mice also exhibited unusually short patterns of sleep, a feature not seen in ordinary mice.
"The implication from the study would be that there is a genetically-wired system in our body to tell us how much sleep do we need," explained Ying-Hui Fu, Professor of Neurology at the University of California in San Francisco, the study's head.
"Yet, we really don't know anything about how this is done. This discovery provides an opportunity for us to begin to probe into the pathway regulating our sleep quantity and need," said Professor Fu, whose study is published in the journal Science.
"It is not clear at the present time how this mutation can lead to short sleep quantity. This is one of the areas that we are pursuing actively," she said.
The scientific evidence suggesting that different people are genetically wired to require shorter-than-average periods of sleep goes back many years.
In 1999, for instance, scientists identified the existence of a gene – or more specifically an inherited mutation within a gene – that appeared to confer something called familial advanced sleep-phase syndrome.
This is an inherited condition where people tend to go to bed early and get up early, which can also happen when people abandon normal sleeping routines, such as at the weekend and when on holiday. People who exhibit this all the time are known as "morning larks", to distinguish them from "night owls" at the other extreme who routinely go to bed late and get up late.
The scientists in this study, led by Christopher Jones of the Howard Hughes Medical Institute, did not actually find the gene or its mutation – they could only show that it must exist in the 29 people from three different families that they had studied.
People with advanced sleep-phase syndrome, however, still sleep for the usual seven and a half to eight hours a night, it's just that their daily routine or "circadian rhythm" is shifted. Scientists believe that genetic mutations can also occur in the genes influencing this aspect of the 24-hour sleep-wake cycle.
Sleep is a product of both circadian rhythm and another controlling factor that, put simply, measures the amount of sleep we have had. When we need sleep, this "homeostatic" mechanism makes us sleepy; when we've had enough sleep, it tells us to wake up.
Professor Fu and her team suspect that the mutation they have found plays a role in the homeostatic mechanism that helps to control the amount of sleep we need. What the latest study tells us is that the actual quantity of sleep needed is partly under genetic control – and that is the result of who our parents were, rather than what we do each day.
Seven ages of sleep
1 Sleep in newborn babies occurs around the clock on an irregular cycle
2 Babies between three and 11 months usually begin sleeping through the night
3 Toddlers between one and three years old need between 12 and 14 hours of sleep and daytime naps
4 Pre-school children begin to have nightime fears and nightmares
5 Problems in the five-to-12 group linked with television and computers
6 Adolescence associated with long sleeping patterns thought to be necessary for development
7 Adult sleep can become progressively difficult and disturbed
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