Sleep

Sleep is a state of which man must have been aware as soon as he started to think about anything at all. The poets have expressed what many of us have felt about it. Coleridge spoke of it as a gentle thing, beloved from pole to pole. Shakespeare called it Nature's soft nurse', tired Nature's sweet restorer, death's counter­feit. The Bible tells us: "He giveth His beloved sleep." Stacey Aumonier was equally emphatic that it was a heavenly gift even if, in one of his best short stories, he dealt with the subject more facetiously.

Until modern times, there were few scientific writings on the subject and the nature of sleep remained something of a mystery. Today, with delicate instruments at our command, to register what is happening in our brains, some of its secrets are being exposed. To a large extent, this new knowledge is still at the clinical stage, comprehensible to the experts but baffling to the layman.

As far as animals are concerned, there has been equal delay. Only in the last decade or so, have inquisitive naturalists bothered to study even the more accessible, outward form of sleep in animals. How and how much do elephants sleep? What do birds do when they sleep? These and other questions of the kind have only just been answered, or are in process of being answered. The results, as we shall see in this volume, can sometimes be surprising.

For many centuries past people have been aware that some animals spend a half, in some instances, as with bats, about three-quarters of their natural lives in sleep. They are the hibernants, the animals that go to sleep for the winter. Our unfolding know­ledge in this field followed a familiar course. At first, it consisted more of folklore and false idea. This was followed by a more enlightened period in which our ideas were still fairly crude. Within the last three decades or so, more refined techniques brought us nearer an understanding. They have also shown that even the commonplace phenomenon we call hibernation is nothing like as simple as we might perhaps have supposed. Certainly, there is much yet to be learned, but the new, yet still elementary knowledge brought to light holds considerable interest.

Most animals need some kind of sleep. The most common types we are familiar with are animals that sleep during the night, known as nocturnal, or animals that sleep during the day, known as diurnal. An obvious example of the former is a Human, and the latter a Bat.

There are animals that, along with their regular daily sleeping patterns, have extremely long periods of sleep such as those that go into hibernation (sleep during the winter) and those that go into estivation (sleep during the summer or dry periods). Some types of bear are known to hibernate, while some lizards and snails go into estivation.

Some animals lay dormant for decades, waiting for the next downpour of rainwater for example.