Legend has it that Napoleon never needed more than four hours of sleep at a stretch. Others only feel fully rested after 10 hours between the sheets. Clearly, individuals vary with respect to how much sleep they need. Indeed, sleep duration is influenced by many factors. Apart from seasonal and other variables, age and sex play a role, as does one’s sleep-wake cycle or chronotype, i.e. whether one is a lark (early to bed, early to rise) or the converse, an owl.