The *Circadian Rhythm* is the “internal clock” of the human body. Many stimuli affect it (Habits, mental state, physical state, genetics, environmental factors, medication, etc).
One of the main components of it is a hormone called Melatonin, produced mainly in the *pineal gland* of the brain. It is mostly influenced by light (hence, using smartphones at night might cause insomnia and other sleep-related issues). Cortisol is also important in the alertness process (Waking up).
This process is also influenced by learning, i.e. the thing we call habit. If you wake up at a certain time for a period of time, the Circadian Rhythm will adjust.
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