What exactly is the body clock?The body clock, or circadian clock, is an internal clock that keeps track of time. Circadian comes from "circa-diem" meaning "around a day". So in a mouse for example it is about 23.5 hours and for a human its about 24.5 hours.
If you are kept in constant darkness your body will still work to around a 24-hour cycle. It is not just being driven by light and dark. It is an intracellular clock so if you look in individual cells they are all able to keep time as well. If you put cells in a dish they will show 24-hour changes in gene expression.
If you don't have any cues, then every day you will get half an hour later – you will drift. So while it is amazing that we have a molecular clock it is a bit like an old-fashioned mechanical clock - either a bit fast or a bit slow. It needs adjusting, you need to check it against the environment every day to keep it at the correct time.
Why is it useful?The advantage of the circadian clock is that it allows anticipation. So rather than passively responding to the environment- it gets light so you get up- you know that the environment is going to change and actually predict it – and of course then change your internal physiology and behaviour to ready yourself for that change in the environment.
Should we listen to our alarm clock or our body clock?Society makes us listen to our wrist watches and we structure our society around time. However, when our internal time is different to external time there is a problem.
A classic example of that is shift work. If you are operating during the nights when your body is telling you to go to sleep ,your cognitive performance will be worse. Even if you do shift work for years you don't just shift your body to being night-active. The reason for that is that light is the primary cue which sets our body clock.