The circadian system keeps us in sync with the 24-hour day. Our body’s internal clock sends signals to many different parts in the body, affecting things like digestion, the release of certain hormones, body temperature, and much more.
One of the main functions of the internal clock is to regulate when you feel alert and when you’re ready to sleep. Scientists have seen, for example, that when people are kept indoors with no sunlight or clocks to tell them what time it is, they wake and sleep in a roughly 24-hour pattern. Plants do this too: Morning glories continue to bloom in the morning even when they’re kept in darkness.
To stay completely in sync with day and night, however, the internal clock accesses external cues. We keep the internal clock in step with the 24-hour day through sunlight, eating, and other activity in the daytime, as well as darkness at night. This process of adjusting or cueing the internal clock with signals from the environment is called entrainment.
In other words, the internal clock is somewhat flexible—if you think about it, that’s why our bodies can keep up with things like daylight savings time and seasonal light fluctuations. This is also why you can travel across time zones and know that your body will eventually come into sync with the local time.