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Why do we wake up from sleep?
Besides environmental stimuli, there are two main processes that regulate sleep.

    The circadian process. The brain has a master circadian clock that keeps an approximately 24-hour schedule. The clock resides in a region of the brain called the suprachiasmatic nucleus (SCN). There are connections from the SCN to several other sleep-regulatory regions of the brain, allowing the SCN to promote sleep and wake and different times of day.

    The sleep homeostatic process. During wake, certain sleep-promoting chemicals accumulate in the brain, including adenosine, nitric oxide, and some cytokines. These chemicals are then cleared from the brain during sleep.

These two processes are integrated by certain sleep-regulatory neural circuits. A circuit in the hypothalamus/brainstem called the sleep/wake switch is thought to play a central role. When the overall signal for wake (from circadian, homeostatic, and other processes) exceeds some critical threshold, this circuit switches abruptly into 'wake mode'. This activates a population of neurons in the brainstem and hypothalamus that express wake-promoting neurotransmitters, flooding the cortex and thalamus with these neurotransmitters.
There are a number of other subsidiary systems that help to express variations in the circadian and homeostatic processes, making sleep and wake states more robust. These include hormones like melatonin, whose release is timed by the SCN's signals to the pineal gland, although melatonin is not required for sleep and wakefulness.


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