The difficulty adolescents have in getting up early in the morning appears to be due to a particular stage of their biological clock rather that to laziness or rebellion. As the light of day diminishes towards evening our body frees melatonin, a hormone that induces sleep. In youngsters this hormone enters blood circulation about two hours later than in adults and children. Adolescents therefore find it easy to stay awake until late at night, but find it difficult to get up early in the morning, because for them it is as if an adult always had to get up at five in the morning. In several schools and universities of the UK a randomized study is underway so see if teenagers and young adults benefit in terms of academic results and health from starting their day at ten (high school) or eleven (university) in the morning. To correct a chronic jet lag, particularly in the age group 14-21 years, study and work-loads that have been set by adults have to be adapted to the biological clock of youngsters.