Throughout the day, a clock ticks inside our bodies. It rouses us in the morning and makes us sleepy at night. It raises and lowers our body temperature at the right times, and regulates the production of insulin and other hormones.

The body’s circadian clock even influences our thoughts and feelings. Psychologists have measured some of its effects on the brain by having people take cognitive tests at different times of day.

As it turns out, late morning turns out to be the best time to try doing tasks such as mental arithmetic that demand that we hold several pieces of information in mind at once. Later in the afternoon is the time to attempt simpler tasks, like searching for a particular letter in a page of gibberish.

Another clue about the clock in our brains comes from people with conditions such as depression and bipolar disorder. People with these disorders often have trouble sleeping at night, or feel groggy during the day. Some people with dementia experience “sundowning,” becoming confused or aggressive at the end of the day.