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Sleep-Wakefulness - National Institute of Mental Health (NIMH)

To be biologically awake is to exist in a state of high cortical activity and environmental responsiveness. This is not a passive state but a precisely controlled sequence of brain events.

Neurotransmitters such as norepinephrine , histamine , serotonin , and orexin (hypocretin) keep the brain alert.

Wakefulness is maintained by a "mutual inhibition" system. When wake-promoting neurons (like those in the lateral hypothalamus) fire, they actively inhibit sleep-promoting areas like the ventrolateral preoptic nucleus (VLPO).