WHAT IS SLEEP?

Sleep is a fundamental necessity of a complex nervous system. It is a homeostatic requirement, similar to need for food and water. The EEG pattern changes profoundly in sleep. There are two phases of sleep dependent on whether or not the eyes move behind the closed eyelids:

• NREM (Non Rapid Eye Movement)
• REM (Rapid Eye Movement)

The initial phases of sleep - NREM sleep - is subdivided into 3 stages:-

• In stage N1 sleep, theta waves begin to be interspersed among alpha pattern in EEG. There is light sleep, aroused by moderate stimuli etc. 
• In stage N2, high frequency bursts called sleep spindles and large amplitude K complexes occasionally interrupt theta rhythm. delta waves first appear along with theta rhythm .
• In stage N3 sleep, as it  continues, the pattern becomes delta rhythm N3, referred to as slow-wave sleep.

NREM sleep normally takes 30 to 45 mins and then process reverses itself; EEG ultimately resumes a small amplitude, high frequency similar to alert, awake state. 

REM sleep is also called paradoxical sleep because even though a person is asleep and difficult to arouse, his or her EEG pattern shows intense activity that is similar to that observed in awake state. In fact, brain oxygen consumption is higher during REM sleep than NREM or awake state.

If uninterrupted, these stages occur in cyclical fashion, tending to move from NREM stages, then back up to N2, and then to an episode of REM sleep. The average total nights sleep comprises 4-5 such cycles, each lasting 90-100 mins. Significantly, more time is spent in NREM during first few cycles, but time spent in REM sleep increases towards the end of undisturbed night.

Skeletal muscle tension, already decreased during NREM sleep, is markedly inhibited during REM sleep. Exceptions include eye muscles which undergo rapid bursts of contractions that give this sleep stage its name. Respiratory muscles are also active during REM sleep, rate of breathing is frequently increased compared to awake, relaxed state.

During NREM sleep, there is pulsatile releases of hormones from anterior pituitary such as GH and GnRH, so adequate sleep is essential for normal growth in children and regulation of reproductive function in adults. Decrease in BP, heart rate and respiratory rate also occur during NREM sleep. REM sleep is associated with increase and irregularity in BP, heart rate and respiratory rate.