Are night owls made or born?

Probably both, though nobody can yet say in what proportion: chronotype has a genetic side, but apart from the most extreme cases it is likely shaped by genes interacting with the environment. A chronotype is a person's propensity to sleep at a particular time during the 24-hour day. Eveningness (most alert in the evening, sleeping late) and morningness (most alert in the morning, sleeping early) are the two extremes, and most people sit somewhere in between: a 2007 survey of over 55,000 people found that the spread follows a normal distribution.

The range is wide. Normal variation covers sleep-wake cycles that are two to three hours later in evening types than in morning types. Chronotype also moves through life: children tend to prefer an earlier sleep period, adolescents a later one, and many elderly people an earlier one again.

Researchers are still working out where chronotype comes from. Studies have found 22 genetic variants associated with it, near genes important for light detection and the circadian clock, yet the causes and regulation of chronotype are said to be undetermined. Apart from the most extreme and rigid cases, it is likely a mix of genes and environment, with cues such as light, feeding, social behaviour and work and school schedules.

One everyday trap is that a clock time means different things at different ages. Horne and Östberg, who co-wrote the morningness-eveningness questionnaire in 1976, noted that a 23:30 bedtime may mark a morning type among students but an evening type among 40 to 60-year-olds.