Scientists agree that sleep is essential for health, and while stages 1 to 4 and REM sleep are important, deep sleep is the most essential of all to feeling rested and staying healthy. The average healthy adult sleeps approximately 1 to 2 hours for every 8 hours of nighttime sleep. While all stages of sleep are necessary for good health, deep sleep offers specific physical and mental benefits. During deep sleep, the body releases growth hormone and works to build and repair muscles, bones and tissues, and the functioning of the immune system.
In addition, slow-wave sleep may be important for regulating glucose metabolism. Elite athletes value slow wave sleep as it helps replenish energy reserves. While a person needs all stages of sleep, deep sleep is especially important for brain health and functioning. Deep sleep helps the brain create and store new memories and improves its ability to collect and remember information.
Ideally, the body will go through four or five of these cycles each night. Waking up at the end of the cycle, when sleep is lighter, may be better to help the person wake up feeling more rested and ready to start the day. Deep sleep is the physiologically deepest stage of sleep. When you enter this stage, your body releases human growth hormone (HGH), a powerful substance that plays a vital role in cell repair.
Accumulated waste products are removed, tissues are repaired and regrown, bones and muscles are formed especially in growing children, and the immune system is strengthened. Stage 2 is the light sleep stage, and we spend about 50% of our sleep hours in it. Your heart and respiratory rates begin to slow as you get comfortable at rest. There is no REM yet, but the brain will begin to produce sleep spindles or rapid bursts of brain activity that regulate sleep.
At this stage, your body and mind begin to relax, so to speak, to travel in and out of consciousness before going into a light sleep. During REM sleep, the eyes move rapidly in different directions, but they don't send any visual information to the brain. Meanwhile, because it's so deep, Stage 3 is also the reason why some of us managed to sleep through a nearby fireworks show or even a storm of thunder and lightning. When scientists study sleepwalking and other nighttime sleep-related disorders, they usually want to focus on Stage 3 of the sleep cycle.
Symptoms of insufficient REM sleep include mental problems, such as memory problems, hallucinations, mood swings, and inability to concentrate. The second stage of non-REM sleep is another, lighter stage of sleep that occurs when the body begins to move into deeper sleep. This stage is also called deep sleep, or delta sleep, due to the brain's production of long, slow waves called delta waves. However, even a physically perfect human body would be of no use if your brain has gone crazy from lack of REM sleep.
Common sleep disorders include insomnia (difficulty falling asleep) and sleep apnea (difficulty breathing during sleep). Meanwhile, at the other end of the age scale, babies can spend half of their sleep cycle in REM sleep. Again, these stages vary from person to person, meaning there's no single time to sleep that's right for everyone. Falling short in slow-wave sleep, in particular, is thought to contribute to insulin resistance and the development of type 2 diabetes and heart disease.