Different people require different amounts of sleep; the more the brain and the muscles are exercised during the day, the more sleep is required. It is a curious circumstance that the lions, tigers, hyaenas, etc, in the Zoological Gardens, which in their wild state roam and feed at night and Bleep by day, when in captivity, reverse this order of things, and feed by day and sleep by night. Man sleeps longer than any of the larger animals, but the great baboon or chimpanzee, the organization of whose brain very closely resembles that of man, will take his six or eight hours' sleep if undisturbed.

The power of intense cold in producing sleep is very great in the human subject; and nothing in the winter season is more common than to find people lying dead in the fields and the highways from such a cause. When Dr. Solander was crossing the mountains which divide Sweden from Norway, in company with Sir Joseph Banks and several other gentlemen, he warned them, saying; "Whoever sits down will sleep; and whoever sleeps will wake no more." Shortly afterwards Dr. Solander was the first who felt an irresistible inclination to lie down, and one of his fellow-travellers, Mr. Richmond, persisted in doing the same, declaring that "he desired nothing better than to lie down and die." Both lay down. Finding it impossible to proceed with them, Sir Joseph Banks and the rest lit fires with brushwood around them; having done which, Sir Joseph endeavoured to wake Dr. Solander, and happily succeeded; but though he had not slept five minutes he had almost lost the use of his limbs, and the muscles were so shrunk that the shoes fell from his feet. He consented to go forward with such assistance as could be given him; but no attempts to relieve Mr. Richmond were successful-he died on the spot.

People of delicate constitutions, in whom the circulation of the blood is languid, are more readily effected by extreme cold than the strong and robust. Some years since, when practising in the south of Illinois, I had been spending the evening at the house of a friend. One member of the family, a gentleman in tolerable health, but not of very strong constitution, had taken a walk to the neighbouring village, only a mile and a-half distant. As the evening was very cold, and he did not return as soon as he was expected, the family became a little alarmed, and part of them started off to meet him. They had not gone far when they found him lying by the fence, cold, stiff, and insensible. They carried him in, and proper measures were adopted, but it was hours before he could be considered quite recovered.

Both the respiration and the circulation are diminished during sleep; the pulse becomes slower and fuller. The vessels of the skin relax; and it has been proved that a person sleeping healthfully and without any artificial means to promote it, will, during an undisturbed sleep in a given space of time, perspire insensibly twice as much as a person awake. The temperature of the body, of course, under such circumstances, falls somewhat below its waking standard. On this account, people more readily take cold asleep than awake. "Therefore," says Dr. Elliotson, "persons cover their heads before going to sleep; and when habit has not overcome the necessity for this, cold is continually caught from its neglect. A draught of air is far more dangerous in the sleeping state, and the back of the body appears less vigorous than the front, as a draught at the back is much more dangerous than in front." It is important, therefore, that during sleep the back should be well covered. Dr. Elliotson adds that "Agues are caught more readily if persons fall asleep." Some persons are more readily affected by cold when asleep than others, and some are so very sensitive that a trifling variation in the covering at night, and even sometimes in the daytime, will produce cold. I once knew a young lady who had a habit of wearing a coral necklace day and night. If by any chance she happened to take it off before going to bed, and forgot to replace it, she invariably took cold.

In severe winter weather, when the poor suffer much from cold, blankets are almost as indispensable to them as food. There is also one interesting and important fact connected with this subject; which is that sleep promotes the cure of all diseases.

How much sleep is necessary for a human being in good health? This must evidently depend a great deal upon the wear and tear to which the system is subjected during waking hours. Age, constitution, climate, occupation, etc, must be taken into consideration. During the first three months of life the time of the infant is divided between sleeping and feeding. As the development of the nervous system, in particular, goes on with remarkable rapidity at this period, the more it sleeps the better. In extreme old age much sleep is also required. The famous Dr. Thomas Parr, who died at the extraordinary age of one hundred and fifty-two years and nine months, latterly slept away the greatest part of his existence. Tall and bulky people are supposed to require more sleep than short and thin people; women more than men; and all animals sleep longer in winter than in summer. In a state of health, during the central period of life, a person actively employed will require-according as the nervous system is more or less fatigued-from six to nine hours sleep. We are informed by his son-in-law, that Sir Walter Scott, both as a young man, and in more advanced age, required "a good allowance of sleep;" and he indulged in it, saying "he was but half a man if he had not full seven hours of utter unconsciousness."

Dr. C. J. B. Williams says that "children, up to the age of six years generally require at least twelve hours of sleep, besides an hour or more in the middle of the day. At about this age, the sleep at noon may be discontinued, but the night sleep can hardly be abridged with advantage, until about the tenth year, and then only to a moderate extent, until the period of puberty, after which it is generally proper gradually to reduce the period of rest to nine or ten hours; and no further diminution is expedient till the cessation of growth, when another hour or two may be taken from it." In more advanced life this extent of sleep is not less serviceable where it can be procured; but at this period the capacity for sleep usually diminishes, and wakefulness or disturbed sleep is a common complaint of old age." "Females commonly stand in more need of sleep than males, and during pregnancy and nursing additional rest is especially demanded. In such cases, too, the loss of sleep is attended and followed by peculiarly injurious results, manifest especially in the nervous system and general nutrition, in the form of mental derangement, impaired vision, deafness, paralysis, palpitation, convulsions, tremours, wasting, diarrhoea, etc. Persons convalescent from acute diseases, or otherwise weakened and reduced, require and generally obtain more sleep than in ordinary health, and it is so efficient an influence in the promotion of recovery, that artificial means are sometimes properly used to procure it.

Much has been said and written about early rising; and there is no doubt that the natural time to rise is when the sun rises, but then the natural time to retire to rest is also when the sun retires -or at sunset. Many people, however, are so habit-ridden, that they insist upon getting up in winter and breakfasting by candlelight, merely because their fathers did it before them. "The laws of nature may be tampered with, but they cannot be subverted; we may step out of the paths she has prescribed, but we cannot go far beyond them with impunity. It needs scarcely any evidence to prove that the day was intended for exercise and the night for repose. That night cannot with impunity be converted into day, has been proved by a variety of observations." Another remarkable circumstance, says an author, has been observed. It is more unhealthy to get up before the sun has risen and burn candles until day-light, than it is to sit up by candle-light after sunset. This is confirmed by Sir John Sinclair, who says: "I have no doubt of the superior healthiness, in the winter time, of rising by day-light, and using candle-light at the close of the day, instead of rising by candle-light, and using it some hours before day-light approaches." Sleep is a provision of nature to restore the exhausted energies of the system, physical and mental. When the nervous energy of the system is restored, the patient will usually wake of himself, and it is not advisable to wake him up unnecessarily.

Too much sleep, however, is debilitating. It weakens the muscular powers, and, in persons predisposed thereto, promotes the accumulation of fat.

There are times, however, in the lives of most people, when, from some cause or other, sleep is an impossibility. Ought narcotics to be taken in these cases? Much will depend on the cause, the nature and the persistency of the sleeplessness. In some cases it is desirable, and in others absolutely necessary to produce sleep artificially, and when really necessary we should not hesitate. Thousands of young children, however, there is no doubt, are annually destroyed by narcotics. The Spanish women are said to employ a more harmless method, and to put their children to sleep by gentle friction up and down the back-bone.