Outline the nature of sleep (8)

Sleep is a state of consciousness where responsiveness to external environment is diminished. It occurs daily as a circadian rhythm with the average adult sleeping for 8 hours out of 24. Within sleep itself, an ultradian rhythm exists with 5 distinguishable stages. EEG machines have helped us to identify these stages objectively. 

Lifespan changes occur within sleep. Infants sleep up to 16 hours a day with 15% spent in REM. After 1 year REM decreases and total sleep time is about 13 hours a day, with an entire sleep cycle taking 45-60 minutes to complete. In childhood 30% of sleep is spent in REM with about 12 hours a day spent sleeping. In adulthood to complete on sleep cycle takes about 90 minutes and consists of both slow wave sleep (SWS) or NREM and rapid eye movement (REM) sleep. Throughout the night the length of time in REM increases with each cycle and NREM decreases. 

The sleep stages consist of stages 1 and 2 which are periods of light sleep and people are easily woken. Alpha waves, characteristic of the awake brain begin to disappear. Slower theta waves, sleep spindles and k-complexes high in amplitude are recorded. Heart rates slows, metabolic rate and temperature drop and brain waves begin to slow down.

Stages 3 and 4 are states of deeper sleep and it is harder to wake someone in these stages. Delta waves become evident in brain activity. These stages are also known as slow wave sleep (SWS). During SWS growth hormones are released to repair proteins and cell synthesis occurs. Metabolic rate, heart rate and temperature are at their lowest points. 

Stage 5 is also known as REM or rapid eye movement sleep. In this stage the body is paralysed but the eyes are moving. This state is often linked to dreaming and brain waves resemble that of someone who is awake.