By Helen Briggs
BBC News science reporter

Jan 28, BBC News

A person's preference for being a "lark" or a "night-owl" is largely determined by genes, a study suggests.

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.

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photo: Genes may influence waking and sleeping patterns (BBC)