What controls sleep

Walker, a neuroscientist and sleep researcher, explains that two main forces govern sleep. One is the circadian rhythm, an internal clock of roughly twenty-four hours that makes us feel alert or sleepy at certain times of day. The other is sleep pressure, driven by a chemical called adenosine that builds up the longer we are awake.

Caffeine works by blocking adenosine’s signal, which is why coffee masks tiredness for a while. People also differ in their natural timing. Some are morning types and some are night owls, and Walker says this is largely inherited rather than a matter of discipline.