A gene that controls how long we sleep has been discovered by scientists explaining why some people have their own internal alarm clock.
Scientists identified a gene called ABCC9 that can reduce the length of time we sleep. The same gene has previously been linked to heart disease and diabetes.
The discovery could explain why light sleepers, such as Margaret Thatcher, are able to get by on just four hours shut-eye a night.
The Europe-wide study saw 4,000 people from seven different EU countries fill out a questionnaire assessing their sleep habits.
Scientists then analysed their answers, as well as participants' genes.
They discovered that people who had two copies of one common variant of ABCC9 slept for "significantly shorter" periods.
Having already established that the ABCC9 gene was also present in fruitflies, the team were able to modify it in the animal and shorten the length of time for which it slept.
study author Dr Karla Allebrandt, from the Ludwig Maximilians University of Munich said: ''The ABCC9 gene is evolutionarily ancient, as a similar gene is present in fruitflies. Fruitflies also exhibit sleep-like behaviour. "
"When we blocked the function of the ABCC9 homolog in the fly nervous system, the duration of nocturnal sleep was shortened."