1. a state of sleep that occurs regularly during a normal period of sleep with intervening periods of REM sleep and is characterized by delta wave brain activity, little dreaming, and a reduced level of autonomic physiological activity
Non-REM sleep is a quiet sleep state. The muscles are relaxed but maintain some tone, breathing is regular, the cerebral cortex generates high-voltage waves, and consumption of energy by the brain is minimal.
— Jerome M. Siegel For most people the few dreams found in non-REM sleep tend to have a rather thought-like character. During REM sleep, on the other hand, dreams occur more frequently and usually have a perceptual vividness …
— Francis Crick et al. NREM sleep … is crucial to memory retention, and to acquiring and refining our motor skills.
— David Kamp — compare rem sleep, slow-wave sleep

2. a state of sleep that occurs regularly during a normal period of sleep with intervening periods of REM sleep and is characterized by delta wave brain activity, little dreaming, and a reduced level of autonomic physiological activity
Non-REM sleep is a quiet sleep state. The muscles are relaxed but maintain some tone, breathing is regular, the cerebral cortex generates high-voltage waves, and consumption of energy by the brain is minimal.
— Jerome M. Siegel NREM sleep … is crucial to memory retention, and to acquiring and refining our motor skills.
— David Kamp — called also orthodox sleep, S sleep, synchronized sleep — compare rem sleep, slow-wave sleep