“The circadian rhythm is an inner clock that follows the twenty-four hours in a day. The master clock sits in the master hormone gland in the middle of the brain, where it regulates the sleep-wake cycle and governs the clocks for the entire body. Each internal organ has its own clock, too, which is somehow coordinated by and with the master clock, so different functions turn on at different times of day. Next to the master hormone gland is a pine cone–shaped gland that makes melatonin, the hormone that induces sleep. The pine cone and the master gland talk to each other, synchronizing their roles to maximize a good night’s rest.”