Scientists have found that each cell of the body has an internal "clock", which can be affected by various genes.

Research in the Proceedings of the National Academy of Sciences shows that skin cells can be used to measure the speed of a person's body clock.

The work could lead to better diagnosis of sleep disorders and conditions such as Seasonal Affective Disorder.

Master clock

It has long been known that the body has a biological clock that regulates sleeping, digestion and brain performance to fit in with the different demands of day and night-time living.