Some people like going to bed early in the evening and waking up at the crack of dawn. Others are most alive after the Sun has set, preferring the darkness of night to the brightness of morning. Research into chronotypes (the propensity to sleep at a particular time during a 24-hour day) shows that people do indeed have stable individual differences in their activity levels at different times of the day. At one end of the continuum are the ‘morning larks’; at the other, ‘night owls’. Although the exact cause of the differences in chronotypes has yet to be unravelled, they are likely to have some genetic influence.