How the body controls sleep
Circadian rhythms are a natural, internal process that regulates the sleep-wake cycle, they repeat roughly every 24 hours, and respond primarily to environmental cues of light and temperature.
Adenosine is an inhibitory neurotransmitter that can act as a central nervous system depressant. It acts to suppress stimulation and promote sleep. When we are awake the levels of adenosine in our brain rise each hour, so the longer we’re awake our need for sleep increases.