Margaret Thatcher famously managed on four hours of sleep a night while Albert Einstein needed 11 - now a study suggests why.
Our need for sleep may be pre-determined by our genes, with one in five Europeans carrying a variation of a gene known as ABCC9.
The gene ABCC9 is involved in sensing energy levels of cells in the body and those with the gene are believed to need almost 30 minutes more sleep each night than those who do not have it.
Scientists at the University of Edinburgh working with colleagues at Ludwig Maximilians University in Munich made the discovery by studying human sleep patterns in more than 10,000 people and genes in flies.
Dr Jim Wilson, at 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."