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.
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.
"It now appears that virtually all cells in the body have their own ticking circadian clock, including skin cells"
The brain has overall control of this time-keeping mechanism, but individual cells appear to have their own "clocks", which can be affected by a series of clock genes.
A German-Swiss team led by Dr Steven Brown of the University of Zurich took skin biopsies from 28 volunteers and grew their cells up in the lab.
They measured how fast the "clocks" in the skin cells "ticked", then compared these results with questionnaires showing whether each subject was an early type ("lark") or a late type ("owl").
They found that the "clocks" in the skin cells matched up with behaviour in most of the subjects.
Some of the remaining volunteers appeared to be suffering from Seasonal Affective Disorder (Sad), a type of winter depression.
Sleep disorders
The work could be used as a basis for the diagnosis of certain sleep disorders, said Dr Simon Archer, Senior Lecturer in Molecular Neuroscience at the Surrey Sleep Research Centre at the University of Surrey, Guildford.
He said previous research at the University of Surrey had shown that genetic differences in clock genes could explain why some people are night lovers and others are early risers.
His group has identified a mutation in a gene known as Period3 that makes someone more likely to be a morning person.
The mutation makes them more prone to tiredness during the day, so they prefer to go to bed early.
Traditional studies of such genes have been carried out in controlled conditions in specialised sleep labs, and these are time-consuming and costly to perform.
"What Steve Brown has developed is a much more economic way of making some of those clock period measurements without having to keep people in a lab for three days," Dr Archer explained.
"He basically takes a sample of their skin cells, keeps them alive in culture and measures the period of the oscillating clock genes."
Molecular clues
The central circadian clock resides in the hypothalamus in the brain, he added.
It keeps in touch with the outside world by means of light signals from the eyes, and then synchronises numerous other peripheral clocks in tissues around the body such as the heart, liver and gut.
"The reason why the Brown experiment works is because it now appears that virtually all cells in the body have their own ticking circadian clock, including skin cells," said Dr Archer.
"So, the skin cell assay is just a way of easily getting a handle on someone's body clock. But in more molecular detail than just knowing if they are a lark or an owl, which can be determined quite easily."
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