Circadian rhythms are "built-in" so that, without time signals from the environment, they keep time at the rate of "about" 24-hour periods. When kept in total darkness, plants and animals continue to behave by the built-in period which is "about" 24 hours. The rhythms are also affected and re-set by time signals such as daylight and the length of the day and the night. It is important that the rhythms be re-set regularly to the natural light/dark cycle. Adult humans of all ages have a circadian rhythm which averages 24 hours and 11 minutes.