Some scientists consider stage 3 and 4 to be one stage. Your body rests during these stages to help cells recover. Your cells produce the most growth hormone here to mend your bones and muscles.
While all this is happening, your muscles are fully relaxed. Mental activity is limited, including dreaming.
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Our molecular clock inside our cells aims to keep us in sync with the sun.
When we disregard this circadian rhythm, we are at a greater risk for illnesses such as diabetes, heart disease, and dementia.
Thomas Edison said that sleep is "a bad habit." Like Edison, we seem to think of sleep as an adversary and try to fight it at every turn. The average American sleeps less than the recommended seven hours per night, mostly due to electric lights, television, computers, and smartphones.
However, we are ignoring the intricate journey we're designed to take when we sleep.
When we fall asleep, the nearly 86 billion neurons in our brain starts to fire evenly and rhythmically. Our sensory receptors become muffled at the same time.
The first stage of shallow sleep lasts for about 5 minutes.
We spend about half our sleeping time in stage 2. It can last up to 50 minutes during the first 90-minute sleep cycle, and less during subsequent cycles.
During this time the brain consolidates the information that has been collected during the waking hours. It makes connections you might not make otherwise. It also curates which memories to keep and which to delete.
We can remain in stage 4 - similar to a coma or brain death - for only about 30 minutes before the brain wakes up and falls back to sleep again.
Even healthy sleepers wake several times a night, but might not be aware of it.
Rapid Eye Movement or REM follows after the four stages of NREM (non-REM) sleep and occupies about one-fifth of total rest time in adults.
During REM, all vivid dreaming takes place. Our internal temperature is at its lowest. Our heart rate increases and our breathing is irregular. Generally, our muscles are immobilized, and we are incapable of physical response, except for our eyes and ears. However, our brain is fully active.
About 80% of our sleeping is of the SWS variety, identified by slow brain waves, relaxed muscles and deep breathing.
Deep sleep is important for the consolidation of memories. New experiences get moved to long-term storage and less important experiences from the previous day get cleared out.