Sancho Panza says: “Now, light on him who first invented sleep! Sleep which covers a man all over, thoughts and all, like a cloak; and is meat for the hungry, drink for the thirsty, heat for the cold, and cold for the hot. Sleep is the current coin that purchases all the pleasures of the world cheap, and the balance that sets the king and shepherd, the fool and the wise man, even.”—“Don Quixote.”  
Sleeping is the one thing that everyone practices almost daily all his life, and that, nevertheless, hardly anyone does as well as when he began. We have improved in our walking, talking, eating, seeing, and in other acts of skill and habit; but, in spite of our experience, few of us have improved in sleeping: the best only “sleep like a child.” It must be that we do not do it wisely, else we should by this time do it well.
 
Even the race of mankind as a whole does not seem to be able to use sleep, to summon it, or to control it any better than man did. We talk much of the need of sleep, and discuss its benefits, but we know neither how to use the of sleep to the best advantage nor how to cultivate it.
 
Yet for ages men have studied the mystery of sleep. We have acquired many interesting facts concerning its variation, and have a number of theories concerning its cause and advantages; nevertheless, science has given us little real knowledge of sleep, and less mastery over it.
 
Mankind has had ever since consciousness began. Advancing knowledge has changed the nature and number of the idols, but it has not destroyed them. The of the present age is “Science,” and men worship it in the degree that it seems to fit their needs. They forget that Science is merely the knowledge of things and persons, arranged and classified, so as to make it available. In its nature it is fallible, for some new phase discovered to-day may show that yesterday’s conclusion was formed from a theory which itself was based on a mistaken . Man has caught a glimpse of something that resembled truth, has stated it, reasoned about it, and finally either established its authority or disproved it through the discovery of the real thing he was seeking. Either result was progress, because man grows, as Browning says, “through at mistake as midway help, till he reach fact indeed.” So there is no need to be disturbed by the conflicting opinions of men of science the purpose or method of sleep. Even the rejected theories have added to the sum of our knowledge, and the field for is still open to all who are faithful in noting and comparing the of Nature, which the scientists call .
 
Most of what we call science has to do with physical or material things. Consequently, we find scientists mainly with what may be called phenomena, those which may be measured or weighed or held in the hand or, at least, pinned down by pressure of thumb or finger.
 
Material Science’s estimate of man is largely by
 
“Things done, that took the eye and had the price;
O’er which, from level stand,
The low world laid its hand,
Found straightway to its mind, could value in a trice.”
This is the almost result of looking upon life as material or physical. We must view life as physical, but not physical only; as mental, but not mental only; as spiritual, but not spiritual only.
 
In studying sleep and its attendant phenomena all these things must be taken into consideration. So slight a thing as fancy may profoundly influence our acts; fancies not attributable to any material source, so and evanescent that the clumsy net of language cannot hold them, may induce sleep or destroy sleep.
 
A review of the theories and conclusions of physicians, both scientific and unscientific, as well as of others who have found the study of sleep of absorbing importance, will find a place in our examination of this vital function of organisms.