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,”
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 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
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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