Your sleep is regulated by an internal body clock, that functions on a 24-hour cycle, also known as circadian rhythm. This biological clock is sensitive to light, time of day, body temperature, and hormones for sleep and awakening. For most of us, our circadian rhythm is set so that we sleep during the nighttime and are awake during the daytime.
When you fall asleep, your brain moves back and forth between different stages of sleep, going from light sleep to deep sleep to rapid eye movement sleep (REM).
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