Dissecting the mechanism of our internal clock: how living organisms tune in to the time of day

As any jet-setter knows, it takes time to adapt to the shifted day-night cycle of a foreign time zone. We have an internal circadian clock that times many physiological and behavioral events on a 24-hour cycle, according to day length. The clock can also reset itself, so we can cope with the seasonal variation in day light hours and the trappings of 20th century living such as shift work and air travel.