A good sleep is needed to give the body a chance to recover, and to have energy for fulfilling the daily responsibilities. This post explores the different stages of sleep, all of them needed to have “nice dreams!”

Sleep Stages (by Kryger et al, 2017)

  • Normal human sleep comprises two states—rapid eye movement (REM) and non–REM (NREM) sleep—that alternate cyclically across a sleep episode.
  • Sleep begins in NREM and progresses through deeper NREM stages (stages 2, 3, and 4 using the classic definitions, or stages N2 and N3 using the American Academy of Sleep Medicine Scoring Manual definitions) before the first episode of REM sleep occurs about 80 to 100 minutes later.
  • Thereafter, NREM sleep and REM sleep cycle with a period of about 90 minutes. NREM stages 3 and 4 (or stage N3) concentrate in the early NREM cycles, and REM sleep episodes lengthen across the night.
  • Newborn humans enter REM sleep (called active sleep) before NREM (called quiet sleep) and have a shorter sleep cycle (about 50 minutes).
  • NREM sleep slow waves are not present at birth but emerge in the first 2 years. Slow wave sleep (stages 3 and 4) decreases across adolescence by about 40% from preteen years and continues a slower decline into old age, particularly in men and less so in women.
  • REM sleep as a percentage of total sleep is about 20% to 25% across childhood, adolescence, adulthood, and into old age, except in dementia.
Wakefulness in sleep usually accounts for less than 5% of the night.
Stage 1 sleep generally constitutes about 2% to 5% of sleep (changes in brain waves).
Stage 2 sleep generally constitutes about 45% to 55% of sleep.  Heart rate and body temperature decrease. Memory consolidation. Bruxism.
Stage 3 sleep generally constitutes about 3% to 8% of sleep. Slow waves, restorative sleep.
Stage 4 sleep generally constitutes about 10% to 15% of sleep. Slow waves, restorative sleep.
NREM sleep, therefore, is usually 75% to 80% of sleep.
REM sleep is usually 20% to 25% of sleep, occurring in four to six discrete episodes.

The normal human adult enters sleep through NREM sleep, REM sleep does not occur until 80 minutes or longer thereafter, and NREM sleep and REM sleep alternate through the night, with about a 90-minute cycle.

  • 75% of sleep is N-REM, 25% is REM
  • N-REM has 4 stages.
    • 1 and 2 are light sleep.  2 is where bruxism is more prevalent
    • 3 and 4  are restorative sleep and is affected by age and chronic conditions such as fibromyalgia
  • REM is suppressed by clonazepam.  It is reduced with age.

Sleep stages:  https://www.verywellhealth.com/the-four-stages-of-sleep-2795920



Kryger MH, Roth T, Dement WC. Principles and Practice of Sleep Medicine. Sixth edition. Elsevier; 2017.