Stage four of non-REM sleep is the deepest stage of sleep, according to Psych Central. Stage four is also called delta sleep because of the deep, slow delta waves of the brain during this part of the sleep cycle, which lasts about 30 minutes.
Some now believe stages three and four of non-REM sleep are one instead of two distinct stages, and it is referred to as slow-wave sleep, according to the Center for Sound Sleep. During this deepest time of sleep, the body's blood pressure drops, breathing slows and muscles relax. Hormones are released during this stage of sleep, and it is when tissue growth and repair occurs and when energy is restored.