Numerous studies have shown that a broad range of activities—both physical and cognitive—vary across the 24-hour day. The blood pressure and heart rate of a human shows striking day–night variations. Simply, an organism is set up to function differently at different times of the day. ‘Time of day matters’ outlines the different human chronotypes—the different preferred waking and sleeping times of humans—and explains concepts such as ‘social jet lag’. There are optimal times of day for physical and cognitive activities, and chronotherapy has also shown that the administration of medication at different times of day can have an important impact on efficacy.