Sleep is a specific state of being when despite the subject appearing to be “turned off”, its body remains active. Not in visible movement, although some minor movements can occur from time to time.
Most regular daily bodily processes are slowed in this stage, with some of them even turned off. Other processes, however, start or even increase their activity. This dynamic occurs primarily revolving around a single goal – to enable and conduct a proper rest.
Physiologically, this means bodily repair and restoration to ensure the organism is fit for another day of functioning.
What Happens When We Sleep?
Based on rigorous analysis of somnologists (i.e. sleep scientists) the existing consensus is that sleep is classified into distinctive, repeatable stages that occur several times during the night. These sleep stages are:
Stage 1: Transition
This is the period between wakefulness and sleep. Muscle activity slows down, and you may experience drowsiness, sudden muscle jerks or sensations of falling, followed by a tic (known as a hypnic jerk), and sometimes even minor hallucinations.
Stage 2: Light sleep
This is the start of true sleep, since in this phase and onward you are unconscious. Body temperature drops, heart rate slows down, and muscles relax further. Eye movements stop.
Stage 3: Deep Sleep
You are the hardest to wake up when in this phase. Blood pressure drops, breathing becomes slower and more regular, muscles are very relaxed, and the body performs main maintenance and repair processes. Growth hormones are released enabling tissue growth. Cell repair and regeneration is increased. Cleaning of the internal tissues occurs, removing various types of cellular waste. Brain activity is minimized, with the brainwave frequency at the slowest pace.
Stage 4: Rapid Eye Movement (REM)
Physiologically, the most active part of sleep, although our body remains completely still except for the eye movement. The heart rate and blood pressure increase, breathing becomes irregular, and the body's major muscles become paralyzed to prevent acting out on dreams that only occur during this stage. The information and learnings from the day are consolidated, filtered, and stored from short to long-term memory. Several new associations are formed as well.
We transition from Stages 2-4 and back in 90 – 120 min sleep cycles. Several of these cycles repeat during the course of the night, yet the amount of time we remain in each phase changes. At the beginning of the night, we spend more time in the deep sleep phase, as the body prioritizes cleaning, maintenance and repair processes. As our body rejuvenates throughout the night, the REM stage share keeps increasing.
A good approximation of the time we spend in each phase during the nightly slumber and how the stages shift is shown in the picture below.
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