What happens when we sleep? During sleep, we cycle through 4 distinct stages - N1, N2, N3, and REM. Each stage is equally important and serves it's own purpose. The two stages that get the most attention, however, are N3 and REM.
During N3 sleep, we experience the growth and repair of cells and tissues, as well as the production of important hormones such as growth hormone. Much of our physical recovery takes place during N3 sleep.
REM sleep is extremely important for cognitive function. It helps us balance our mood, facilitates learning, and helps us with memory consolidation. In fact, REM sleep is such a crucial process that our brain actually keeps an internal "tracker" on REM sleep duration. In extreme cases, if a person is chronically deprived of REM sleep, they will actually begin to experience REM sleep while they're awake in the form of hallucinations.
All of this is only scratching the surface of the true importance of optimal sleep, and the many physiological and psychological processes that require optimal sleep and recovery.
The text below has been taken directly from the NINDS (National Institute of Neurological Disorders & Stroke), and describes each stage in thorough detail. Source link: https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep#2
"There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages). Each is linked to specific brain waves and neuronal activity. You cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.
Stage 1 non-REM sleep is the changeover from wakefulness to sleep. During this short period (lasting several minutes) of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brain waves begin to slow from their daytime wakefulness patterns.
Stage 2 non-REM sleep is a period of light sleep before you enter deeper sleep. Your heartbeat and breathing slow, and muscles relax even further. Your body temperature drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity. You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.
Stage 3 non-REM sleep is the period of deep sleep that you need to feel refreshed in the morning. It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to their lowest levels during sleep. Your muscles are relaxed and it may be difficult to awaken you. Brain waves become even slower.
REM sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. As you age, you sleep less of your time in REM sleep. Memory consolidation most likely requires both non-REM and REM sleep."
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