Is a quick "power nap" really as good for you as a full night's sleep?
Don't be misled by headlines. The emphasis was on performance, not health. Researchers at Harvard University in Massachusetts tested volunteers with a visual learning task - and those who stayed awake all day performed less well at the end of the day. Those who had dozed off for an hour, or an hour and a half, on the whole did significantly better - but only if they showed evidence of rapid eye movement or REM sleep, linked with dreaming.
What has dreaming to do with it?
Nobody is sure, but dreaming seems to be a way of unconsciously consolidating or sifting memories. Psychologists claimed last year that they could "eavesdrop" on the dreams of rats that had been trained to run around a maze - and found them dreaming about their daytime challenges. Experiments with young zebra finches have shown that sleeping songbirds replay and perhaps rehearse the songs they normally learn by listening to adults. And experiments with students have shown, repeatedly, that they perform better when invited to "sleep on it".
So dreamless sleep is not so vital?
Margaret Thatcher, the former prime minister, claimed to need only four hours of sleep (although it was later revealed that she also needed vitamin injections in her buttocks). Thomas Alva Edison ate sparingly and claimed to need little sleep, but did keep a camp bed in the laboratory, to which he would retreat in the course of inventing the phonograph, electric light bulb and so on. But although anyone may benefit from a post-prandial doze, most people seem to need a reasonable night's sleep.
How much is reasonable?
The University of Pennsylvania school of medicine this year tested healthy young volunteers with regimes of four hours a night, and six hours a night. Those woken up after four hours reported feeling "only slightly sleepy" but in fact, they performed badly in standard psychological tests. People seem to need at least six hours a night as a routine: restricted sleep "can seriously impair our neurobiological functioning", the team reported. Studies of shift workers, hospital doctors and surgeons, parents of young children and military personnel have repeatedly shown that sleep deprivation impairs performance. Researchers claim that an estimated 15-20% of all motor vehicle crashes could be linked to insufficient sleep.
Wakeful nights can seriously damage your health?
People who sleep well end up with higher glucose concentrations in the blood and lower concentrations of thyrotropin, the protein that regulates the release of thyroid hormones. They score well with other indicators of health and capacity to perform. Lack of sleep has been linked to high blood pressure, weight gain, gastrointestinal disorder and cluster headaches. In the more developed countries, average sleeping time has fallen from nine hours to 7.5 hours. Teenagers, however, sleep 8.5 hours during the week, and will snore on for more than 9.5 hours at weekends if left undisturbed.
Can you have too much sleep?
Paradoxically, yes. A team at the University of California, San Diego, studied the records of more than a million adults, aged from 30 to 102. They sifted through variables such as diet, age and exercise, and then did some calculations for death rates and sleep patterns between 1982 and 1988. The discoveries were surprising. Death rates were lower for those who recorded six to seven hours a night. Those who slept more than eight hours or less than four hours a night had a significantly increased death rate. Occasional insomniacs were not more likely to die early - but the researchers observed that quite often people who complained of insomnia in fact had "normal" amounts of sleep. However, people who took sleeping pills were more at risk. This could have been due to a link between chronic insomnia and clinical depression.
Is there a simple answer to insomnia?
Sleeplessness is calculated to cost the US £25bn a year. The normal prescription is a light meal, gentle exercise, a soporific book and a quiet room. If these don't work, there are mental strategies. Counting sheep, the traditional solution, may not be the best. A cognitive scientist at Oxford University last year divided 50 volunteer insomniacs into three groups: one counted sheep, a second was invited to concentrate on relaxing thoughts of holidays or waterfalls, a third was just told to put up with it. People who contemplated twilight in the Tyrol, or lavender fields in Provence, nodded off 20 minutes earlier than usual. The sheep-counters and the ones who lay there and seethed actually stayed awake longer than they might have done.
Why do people sometimes sleep badly?
Jet lag or other body-clock disruption, shift work, anxiety and sudden switches in altitude all play a part. Astronauts, liable to suffer from all of these conditions at the same time, can have difficulties. Observation of sleeping patterns aboard Mir and the international space station suggests that the human body really is tied to a 24-hour rhythm, linked to the earthly patterns of day and night. After 90 days in space, the endogenous circadian pacemaker - the part of the brain that tells you when to nod off - begins to wane: sleep quality and quantity deteriorates. There are more serious sleep disorders. Obstructive sleep apnoea has been linked to atrial fibrillation and then heart failure, for instance. More tragically, an inherited condition called familial fatal insomnia has been identified in 24 extended families worldwide.