It has been stated many times that a form of motor paralysis sets in during sleep. Yet we all know of the many motions performed by every sleeper, turning from side to side, drawing or pushing away the bed clothes, removing stimuli applied to the face, talking, not to mention, of course, sleep walking.
Sleep does not even mean complete muscular relaxation, for sentinels have been observed who could sleep standing; some people sleep sitting up in their chairs. Many animals, birds, bats, horses, sleep in positions which make muscular relaxation impossible; when their balance is disturbed by an observer, they re-establish it without awaking. Sleeping ducks keep on paddling in circles to avoid drifting against dangerous shores, etc.
In other words, there is not a part of our body which ceases in sleep to perform its specific work. Our lungs continue to breathe, our heart to send blood to all parts of the body, our glands secrete various chemicals; we hear, smell and to a certain extent, see. The lowering of our eyelids is simply a half-conscious effort to remove sight stimuli. Our nails and hair continue to grow, although, for that matter, they do so for some time even after death.
Finally our mental activity does not cease during sleep. Wake up a sleeper at any time and he will awaken _from a dream_. He may not be able to tell that dream but he will know for sure that, not only was he dreaming, but had been dreaming for a long while before awaking.
Wherein, then, does sleep differ from waking life?
Solely in the form of our mental activities.
Sleep is not as Manacéine, the author of the most complete book on sleep, stated: the resting time of consciousness. We do not withdraw our attention completely from the environment in sleep.
When we make up our minds, for instance, to wake up at a certain time, we seldom fail to carry out our purpose. Which does not mean that we are suddenly aroused out of our unconsciousness by something within ourselves, but more probably that our attention has been concentrated all night on certain stimuli indicating time, distant chimes, activities taking place at a definite hour, and which we had noticed unconsciously, although they may have escaped our conscious attention. It has even been suggested that as respiration and pulse are more or less constant in rest, they are used by the organism as unconscious time-registers. This is possibly one of the phenomena due to the activity of the pituitary body in which may reside the "sense of time" and which controls all the rhythms of the body.