The prolonged sleep of some animals in the winter, called “hibernation,” seems to be closely similar to ordinary sleep, but is set up by the depressing action of continuous cold instead of by the daily recurring quiescence of night and by the exhaustion due to the day’s activity. Many animals—such as the marmot and dormouse, the frog and the snail—exhibit this winter sleep. It has been found by experiment that even in midsummer the dormouse can be made to “hibernate,” by exposing it artificially to a low temperature, and hibernating animals can be roused from their long sleep by bringing them into warmth. During the winter sleep hibernating animals take no food, the pulse is slowed down, and the body temperature falls. The scattered fat of the body, and fatty matter and other material stored in special structures called “hibernating glands,” are oxidised and slowly consumed during this period, which may last for three or even four months. The animal on waking is often in a very emaciated condition. It is undoubtedly the case that the human natives of high latitudes (such as the Norwegians), where there is no night in full summer, and where there is prolonged darkness in winter, have acquired the habit of keeping awake for many days in succession in summer, whilst making up for the loss of sleep by excessive indulgence in it during the winter. It is by no means clear how far man is capable of resisting the demand for recurrent daily sleep without injury to health. Undoubtedly many men are compelled by their avocations to sleep by day and wake by night. The length and duration of “spells of sleep” and the power to sleep little or not at all at one season, and almost uninterruptedly at another, without injury to health, are matters of habit, occupation, and circumstance. We have no ground for saying that every man “ought” to sleep eight hours or more per diem, or, on the contrary, for insisting that he should only sleep five or less. All depends on what he is doing when he is awake, and what other people are doing (so as to disturb him) when he is asleep; and we do not even know whether ten or twelve hours’ sleep would injure a man, were he able to take it, nor can we suggest how it would injure him supposing it did not interfere with his feeding and exercise.