ABSTRACT
This chapter examines the current knowledge about sleep from different perspectives. It focuses on circadian rhythms, sleep stages, current ideas about memory, how sleep changes as people age, the influence of light on their sleep, tiredness, how social cues impact on their sleep, and then how physical and psychological insults can reduce the quality and quantity of their sleep. There are three states of consciousness, which are all very different from one another. They are: Awake, REM sleep and non-REM sleep. Adults have a circadian rhythm of ninety minutes, and this is very regular. Even in people in the advanced stages of dementia, this endogenous rhythm clicks along continuously and many of their functions are controlled and influenced by it. To return to REM sleep and non-REM sleep for a moment, it is worth mentioning memory and the concept of a "forgettery".
