Stages of Sleep

Human sleep comprises five recurring stages, during which there is a change in the quality of sleep, leading to unique effects on the mind and body. This can be monitored through polysorrmography in sleep laboratories which provide data on the electrical and muscular states during sleep. The period of Non-Rapid Eye Movement (NREM) comprises Stages 1-4 and lasts between 90 and 120 minutes, with each stage ranging between five and 15 minutes. The stages occur cyclically, repeating themselves through the night, and each sleep cycle lasting longer than the previous one. Typically, most people complete five cycles during one night's sleep.

Stage-1 : Light Sleep

Muscles relax and body temperature drops. The person drifts between sleep and wakefulness and is still conscious of being awake.

Stage-2 : Transition

The person’s heartbeat and breathing slow down, as does electrical activity in the brain. This is the stage before deep sleep begins.

State-3&4 : Slow Wave or Delta Sleep

Also known as deep sleep. the body does not respond to sound or touch. It is the hardest to wake up a person in this stage.

Stage-5 : Rem or Rapid Eye Movement

Eyes and face may twitch but the muscles are paralysed. Heartbeat and respiration become erratic and cerebral activity increases. Intense dreaming begins but people cannot enact dreams as the muscles are immobilised. The first REM stage lasts 10 minutes, increasing each subsequent time.

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