I really need to get some sleep.
Sleep is a naturally recurring state of mind and body
It's really hard to define sleep but that is a solid one.
Sleep
Sleep is a naturally recurring state of mind and body, characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity and inhibition of nearly all voluntary muscles during rapid eye movement (REM) sleep, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but more reactive than coma or disorders of consciousness, sleep displaying very different and active brain patterns.
Sleep occurs in repeating periods, in which the body alternates between two distinct modes: REM sleep
distinct brain regions.
Sleep function.
The need and function of sleep are among the least clearly understood areas in sleep research. When asked, after 50 years of research, what he knew about the reason people sleep, William C. Dement, founder of Stanford University's Sleep Research Center, answered, "As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy." It is likely that sleep evolved to fulfill some primeval function and took on