Some people may need more sleep than others because they carry a certain gene variant, scientists have discovered.
The study found that those with a variation of the gene ABCC9 need almost 30 minutes more sleep each night than those who do not have it.
One in five Europeans carry a variant of ABCC9, which is involved in sensing energy levels of cells in the body. Scientists at the University of Edinburgh and Ludwig Maximilians University in Munich made the discovery by studying human sleep patterns and genes in flies. They found that flies without ABCC9 slept for three hours less than normal.
The scientists said that in humans, presence of the gene variant explained the need for some people to sleep longer than eight hours, the length of time the study classed as average.
Dr Jim Wilson, of the University of Edinburgh's Centre for Population Health Sciences, said: "Humans sleep for approximately one-third of their lifetime.
"A tendency to sleep for longer or shorter periods often runs in families despite the fact that the amount of sleep people need can be influenced by age, latitude, season and circadian rhythms. These insights into the biology of sleep will be important in unravelling the health effects of sleep behaviour."
He added: "Too much or too little sleep is associated with health problems such as high blood pressure, obesity and heart disease. Trying to understand the relationship between sleep and these diseases is a very important area."
More than 10,000 people throughout Europe took part in the study and provided information on how many hours they sleep each night, along with a blood sample to analyse their DNA. Their sleep was measured on days classed as 'free', when they did not need to get up for work, take sleeping pills or work shifts.
Researchers also found that seasonal changes in daylight at different latitudes appear to have an effect on how long people sleep, but more research is needed to fully understand how time of year and geography are involved.
The paper, published in Molecular Psychiatry, looked at data from Orkney, Croatia, the Netherlands, Italy, Estonia and Germany.