People who like a lie-in may now have an excuse - it is at least partly down to their genes, according to experts.Experts, who studied more than 10,000 people across Europe, found those with the gene ABCC9 need around 30 minutes more sleep per night than those without the gene.The gene is carried by one in five Europeans, they say in their study, published in Molecular Psychiatry.The researchers said the finding could help explain "sleep behaviour".Over 10,000 people took part, each reporting how long they slept and providing a blood sample for DNA analysis.People's sleep needs can differ significantly.At the extreme, Margaret Thatcher managed on four hours of sleep a night while Albert Einstein needed 11.Fruit fliesPeople from the Orkney Isles, Croatia, the Netherlands, Italy, Estonia and Germany took part in the study.All were asked about their sleep patterns on "free" days, when people did not need to get up for work the next day, take sleeping pills or work shifts.When the researchers from the University of Edinburgh and Ludwig Maximilians University in Munich compared these figures with the results of the genetic analysis, they found those with a variation of a gene known as ABCC9 needed more sleep than the eight-hour average.They then looked at how the gene works in fruit flies, who also have it and found flies without ABCC9 slept for three hours less than normal.The gene ABCC9 is involved in sensing energy levels of cells in the body.They say this opens up a new line of research in sleep studies, and it is hoped that future work could establish exactly how this gene variant regulates how long people sleep for.Dr Jim Wilson, from 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."Sleep expert Neil Stanley said around half a dozen genes had been linked to sleep patterns.He added: "It's interesting to know about these genes, but in a way our genes are an irrelevance unless you were actually to obey them - but none of us do that."