It has long been clear that different individuals can have markedly different sleep patterns. Most of us display some degree of "morningness" or "eveningness": we are either "larks" or "owls". This is largely a matter of genetics. Natural early risers are born that way, and so are people who prefer to get up late and stay up deep into the night.

In December 2004, Till Roenneberg at Ludwig Maximilians University in Munich, Germany, suggested that, in addition, our degree of morningness or eveningness changes with age. Roenneberg's idea was based on his studies of the sleep habits of more than 25,000 people aged between 8 and 90. This revealed that, though young children tend to be larks, from the age of about 14 they typically begin going to sleep (and waking) later and later (Current Biology, vol 14, p R1038). He also found that around the age of 20, the trend reverses and people start sleeping at steadily earlier times. Women reach this turning point at an age of 19.5 years on average, while for men it comes at 20.9 years. "Most young children are morning-type people - they wake early and are at their most alert in the morning," he says. "Around the onset of puberty, there is an alteration in the body's clock so that teenagers are shifted forwards, becoming evening-type people." They perform better later in the day, in the afternoon or evening, and find it hard to sleep before 11 pm, midnight or even 1 am.

People with the long genes tend to be night owls - late to bed and late to rise. After staying awake all night, this group only scored half as well on cognitive tests as their "short" counterparts.