During sleep, most of the body's systems are in an anabolic state,
helping to restore the immune, nervous, skeletal, and muscular systems;
these are vital processes that maintain mood, memory, and cognitive performance, and play a large role in the function of the endocrine and immune systems.
The internal circadian clock promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.
The advent of artificial light has substantially altered sleep timing in industrialized countries.
