How does the biological clock work?
Here’s the short answer — a link to the long version iscoming up. In brief: in a particular region of the brain called the hypothalamus (here is a Brain Tour) contains a collection of neurons which function as a clock. These neurons “know” when it is morning, and start many of the biologic rhythms your body depends on during the day. Likewise, they know when it is nighttime: they then turn off daytime rhythms, and turn on nighttime functions including the secretion of melatonin which is associated with sleep. Therefore your inclination to sleep is very directly controlled by the biological clock. Likewise, your daytime wakefulness also depends on the clock. The individual molecules associated with this biological clock function have been worked out in remarkable detail. Originally this was done using the biological clock in a fruit fly has the model, but the mechanism in humans turns out to be remarkably similar. A recent study showed that by manipulating one of the genes responsible
