They say the early bird catches the worm, but night owls may be missing far more than just a tasty snack. Researchers have discovered the first physical evidence of structural brain differences that distinguish early risers from people who like to stay up late. The differences might help to explain why night owls seem to be at greater risk of depression.

Around 10 per cent of people qualify as morning people or larks, and a further 20 per cent are night owls – with the rest of us falling somewhere in between. Your lark or night owl status is called your chronotype.

Previous studies have suggested that night owls experience worse sleep, more tiredness during the day and consume greater amounts of tobacco and alcohol. This has prompted some to suggest that they are suffering from a form of chronic jet lag.

To investigate further, Jessica Rosenberg at RWTH Aachen University in Germany and colleagues used diffusion tensor imaging to scan the brains of 16 larks, 23 night owls and 20 intermediate chronotypes. They found a reduction in the integrity of night owls' white matter – brain tissue largely comprised of fatty insulating material that speeds up the transmission of nerve signals – in areas associated with depression.

"We think this could be caused by the fact that late chronotypes suffer from this permanent jet lag," says Rosenberg, although she cautions that further studies are needed to confirm cause and effect.

Skewed body clocks

Although the team controlled for tobacco and alcohol use, it's possible that gene variants that skew people's body clocks towards nocturnal living could affect the structure of the brain. It's also not clear whether the structural changes have any implications for people's health.

"It's interesting that there are individual differences, but we need to understand what is causing them and find ways of creating environments in which those differences can be attenuated," says Derk-Jan Dijk, director of the Surrey Sleep Research Centre in Guildford, UK, who was not involved in the study.

Rosenberg suggests that people's work schedules should change to fit in with their natural sleep patterns, but Djik says there may be an easier way. For example, research published last month suggests that night owls who cut their exposure to artificial light and boosted their exposure to sunlight found their body clocks shifted towards earlier waking and sleeping (Current Biology, DOI: 10.1016/j.cub.2013.06.039).