1. Stages of Sleep
There are 5 stages of sleep, stage 1-3 NREM (non rapid eye movement), REM (rapid eye moment) sleep and W (waking). For the average adult, a single sleep cycle begins with stage 1 sleep and ends with REM sleep. Adults spend nearly half of their sleep time in stage 2, about 20% in REM and the other 30% divided between the other three stages (light and deep sleep)
For the average baby, there are only two sleep stages; active (or equivalent of REM sleep in an adolescence) and quiet. Active sleep comes first followed by quiet sleep.
Figure 2: REM represents 50% of sleep time in newborns
2. Circadian Rhythm
The circadian human biological clock is located in the tiny suprachiasmatic nucleus (SCN) within the hypothalamus portion of the brain. The biological clock drives the daily expression of vital homeostatic including feeding, drinking, body temperature, and neurohormone secretion into near 24 hour oscillations or circadian rhythms.
The biological clock is driven by the SCN and is triggered by light. The SCN receives the signal information on the length of day and night from the retina of the eyes, interprets and passes the information onto the pineal gland located in the epithalamus near the centre of the brain. The pineal secretes the hormone melatonin, controlling the daily night-day cycle which in turn, allows the entrainment of the circadian rhythms.
The circadian rhythm relies on two main hormones:
Cortisol – a hormone that keeps you alert
Melatonin – the hormones of drowsiness
These hormones levels are influences by light, which subsequently affects the circadian rhythms which drive the homeostasis system