25 September 1999

AT THE end of a century when the mysteries of life have fallen one after
another to the advance of molecular biology, it is reassuring to find that we
still have one big puzzle for the next century’s scientists to get their teeth
into. Even more delightful, it’s a puzzle that can be expressed in just four
words: “Why do we sleep?”

Nobody knows the answer, despite fifty years of intensive research. Everyone
knows, of course, that we fall asleep because we are tired and somehow sleep
restores our sense of wellbeing. But what exactly does sleep revive and why? And
why can’t we do without it?

A popular view is that we sort out the day’s experiences and reinforce
memories during sleep, perhaps through a subtle transfer of information between
different parts of the brain
(see p 26).
But the trouble with such theories is
that they are both too weak and too strong. On the one hand, they don’t explain
the absolute need we have for sleep. If totally deprived of sleep, animals don’t
just become a little confused about remembering their way through a maze, they
actually die—and almost as quickly as if deprived of food (see New
Scientist supplement, 26 April 1997). On the other hand, the theories don’t
really explain why we can handle a few nights without sleep and not suffer
serious mental impairment. If you don’t count a tendency to bite everybody’s
head off, that is.

Where can we look for an answer? The usual place to start when tackling big
“what for” questions is in our own evolutionary past. Much of our physiology and
behaviour can be understood by studying other animals, but when we turn to sleep
the plot merely thickens.


It is essential, of course, to distinguish between the two major phases of
sleep—slow-wave sleep (SWS) and rapid eye movement (REM) or dreaming
sleep. Dreaming is restricted to the higher animals— mammals, birds and
some reptiles—and thus seems likely to have a cognitive function. But even
that rule has its strange exceptions. Although we might forgive the echidna, one
of the most primitive mammals, for not indulging in REM sleep, why is it that
dolphins and whales also seem not to dream?

How long animals sleep is also very variable. Bats sleep for up to 20 hours a
day. Both the giant anteater and the giant sloth also put in 18 hours, but the
anteater dreams for around 7 hours while the sloth makes do with 70 minutes. Is
chasing ants really such a cognitive strain? Lions can sleep for days on end.
But a shrew sleeps for only a few hours.

Generally speaking, animals that are secure from predators sleep a lot, while
those whose lives are at risk sleep little. That could suggest that sleep is
designed simply to conserve energy. Evolution follows the philosophy of the
couch potato—if you are well fed and secure, just take it easy. But even
here, there are exceptions. Elephants have few predators but sleep little. The
Indus dolphin lives in a dangerous environment but manages to clock up 7 hours a

Perhaps all we can really be certain of is that although the amount and type
of sleep is in some way related to lifestyle, no higher animal can survive
without at least some sleep. Which suggests that the roots of sleep run very
deep. Indeed, all animals go though cycles of activity and rest, even the humble
amoeba. Sleep has always seemed to be something more complicated. But perhaps,
after all, it is not—a view recently expressed Giulio Tononi of the The
Neurosciences Institute in San Diego and one of the leaders in the field, in the
question: “Can single neurons sleep?” In other words, we still don’t really even
know whether sleep is a local or a global phenomenon of nervous systems.

If global, then we can expect neurobiologists to crack the problem of what
sleep is for. If local, then molecular biologists will find the answer inside
single cells. And if sleep is just nature’s way of stopping us wasting precious
energy then it will be behavioural ecologists who strike it lucky with the right

Whatever the outcome, it’s good to know that we have a deep mystery left.