Slow-wave sleep (SWS)
SWSSlow-wave sleep is the deepest stage of non-REM sleep, identified by large, slow brain waves called delta waves on an electroencephalogram.
It dominates the first half of the night and is the stage from which people are hardest to wake. During slow-wave sleep, heart rate, breathing and blood pressure fall, growth hormone is released, and the brain clears metabolic waste and consolidates certain memories. It is often called deep sleep and is thought to account for much of the physically restorative effect of a night's rest. The amount declines steadily with age.
Sleep laboratories measure it as a percentage of total sleep during polysomnography, and consumer wearables estimate it less precisely from movement and heart rate. Reduced slow-wave sleep is seen in insomnia, sleep apnea and some mood disorders. Cannabinoids have been observed to alter slow-wave sleep in some studies, and such changes are not the same as a clinical benefit.