Sleep is a state of psychomotor arrest with an increased arousal threshold and sensory disconnection from the environment.
As humans we spend 1/3 of our lives sleeping, and sleep is not merely a passive state of rest.
Normal sleep comprises 2 major states; NREM Sleep and REM Sleep (~25%)
NREM Sleep includes – stage 1 (5%), stage 2 (~50%) and stage 3 (~ 25%, deep sleep)
REM sleep includes skeletal muscle atonia and most of the dreaming occurs during this stage
Each sleep cycle is 90-120 mins (NIH, 2022)
With age, N3 sleep decreases to 10-15% for over 65 yrs of age
Circadian Rhythm and Process S
Process S: accumulation of adenosine during wakefulness promotes sleep by building homeostatic sleep pressure
Circadian Rhythm: what drives wakefulness and peak cognitive ability, declines as day progresses
The widest gap between Process S and Circadian rhythm = Sleep pressure that drives sleep
Both these process are regulated in the hypothalamus
Process S / Hemeotstatic switch is located in the Ventrolateal preoptic nucleus (VLPO)
Circadian rhythm is influenced by retinal cells that perceive light, send signals to Suprachiasmatic nucleus (SCN) which in turn regulates sleep (melatonin production), temperature and endocrine output
Hence it is appropriate to say, light is the pacemaker of sleep
Sleep and Cognition
Deep sleep (N3) is responsible for glymphatic clearane of amyloid-beta and tau (Nature, 2026;17:715)
Losing even one night of sleep may lead to risk of beta amyloid and tau buildup in the brain
Low N3 = higher risk of Dementia
References:
- Sleep Phases and Stages. NIH. Accessed July 2026.
- Dagum, P., Elbert, D.L., Giovangrandi, L. et al. The glymphatic system clears amyloid beta and tau from brain to plasma in humans. Nat Commun 17, 715 (2026).