sample essays of IELTS

17 Evolution of sleep

Sleep is very ancient. In the electroencephalographic sense we share it
with all the primates and almost all the other mammals and birds: it may
extend back as far as the reptiles.

There is some evidence that the two types of sleep, dreaming and
dreamless, depend on the life-style of the animal, and that predators
are statistically much more likely to dream than prey, which are in turn
much more likely to experience dreamless sleep. In dream sleep, the
animal is powerfully immobilized and remarkably unresponsive to external
stimuli. Dreamless sleep is much shallower, and we have all witnessed
cats or dogs cocking their ears to a sound when apparently fast asleep.
The fact that deep dream sleep is rare among pray today seems clearly to
be a product of natural selection, and it makes sense that today, when
sleep is highly evolved, the stupid animals are less frequently
immobilized by deep sleep than the smart ones. But why should they sleep
deeply at all? Why should a state of such deep immobilization ever have
evolved?

Perhaps one useful hint about the original function of sleep is to be
found in the fact that dolphins and whales and aquatic mammals in genera
seem to sleep very little. There is, by and large, no place to hide in
the ocean. Could it be that, rather than increasing an animals
vulnerability, the University of Florida and Ray Meddis of London
University have suggested this to be the case. It is conceivable that
animals who are too stupid to be quite on their own initiative are,
during periods of high risk, immobilized by the implacable arm of sleep.
The point seems particularly clear for the young of predatory animals.
This is an interesting notion and probably at least partly true.