What sleep actually does

For much of human history, sleep was considered wasted time. Neuroscience has overturned that view comprehensively. Sleep is one of the most metabolically active and functionally critical states the brain enters.

The stages of sleep

A healthy night cycles through roughly 90-minute stages:

  • Light sleep (N1, N2): transitions in and out of deeper stages; N2 features sleep spindles thought to play a role in memory consolidation
  • Deep sleep (slow-wave, N3): the most physically restorative stage; growth hormone is released; the glymphatic system activates
  • REM sleep: brain activity resembles waking; most vivid dreaming occurs; essential for emotional processing and declarative memory

The glymphatic system

One of the most significant sleep neuroscience discoveries of recent decades is the glymphatic system: a network of channels around brain blood vessels that, during deep sleep, flushes cerebrospinal fluid through the brain, clearing metabolic waste including beta-amyloid and tau proteins associated with Alzheimer's disease.

The cost of too little sleep

Chronic sleep deprivation (routinely getting less than seven hours) is associated with increased cardiovascular disease risk, impaired glucose regulation, compromised immune function, reduced cognitive performance and increased dementia risk in later life.