For most, around 30% of our human existence is spent in unconsciousness. Yes sir, that's right. My textbook of medical physiology defines this unconsciousness as sleep, and lets one know that a person can be aroused from this state by sensory or other stimuli. [This state is to be distinguished from coma, which is a state of unconsciousness from which the individual cannot be aroused.]
It is recognized that there are multiple stages of sleep, ranging from a very light sleep to a very deep sleep. It has also been shown that a person goes through two different types during this period of unconsciousness. One type is called "slow wave sleep", and the second is called "REM sleep". [Stands for rapid (R) eye (E) movement (M) sleep.] Each night approximately 75% of sleep is the slow way type. It is during REM sleep that dreaming is usually associated.
Now sleep produces its effects on the nervous system itself, and on other structures of the body. It is part of the cycle called "sleep-wakefulness". How about that, we all share this cycle [consciousness to unconsciousness] on a daily bases. It is a common thread of our human existence.
My text is:
Textbook of Medical Physiology, Eighth Edition, by Arthur C. Guyton. It was published by W.B. Saunders Co., 1991. Pages 659 - 663 discuss this topic. It was the 1971 edition that accompanied me through medical school.