Researchers have discovered the first physical evidence of structural brain differences that distinguish early risers from people who like to stay up late. 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. The brain differences might help to explain why 'night owls' seem to be at greater risk of depression than 'larks'. Brain scans have shown 'night owls' have a lower quality of white matter (tissue largely comprised of fatty insulating material that speeds up the transmission of nerve signals) in areas of the brain associated with depression. 'Night owls' should try to adjust their sleep patterns if they wish to avoid mental health problems.