Pupils are suffering with depression and low self-esteem because of a lack of sleep, according to US research.
Children need to ensure they get enough rest, academics say
A Massachusetts University study of 2,259 children aged 11 to 13 in Illinois found those who lacked rest also gained worse academic grades.
Girls found it harder to get enough sleep, possibly because they entered puberty earlier, creating a greater need for it.
They also woke up earlier, spending longer on getting ready and chores.
Psychologist Jean Rhodes said: "Elevated levels of depression and drops in self-esteem are seen as inevitable hallmarks of adolescence.
"Yet these results suggest that such changes are partially linked to a variable - sleep - that is largely under individual, parental and even school control."
The amount of sleep gained declined as pupils advanced through school, researchers found.
The findings are published in the US medical journal Child Development.