"This particular population is more likely to experience insufficient sleep, and their functioning is more negatively affected by lower sleep quality," said one of the researchers Sylvie Mrug, psychology professor at the University of Alabama at Birmingham in the US.

The researchers examined two dimensions of sleep – sleep duration and sleep problems from the perspectives of adolescents and their parents, as well as the levels of the stress hormone cortisol before and after social stress.

Eighty-four adolescents with an average age of approximately 13 took part in the study. In stress tests, the researchers found that cortisol levels that indicate increased stress response are higher in adolescents with sleep problems.

"The result of higher cortisol levels in adolescents experiencing sleep problems was exactly what we expected to see," Mrug said. "We were, however, surprised that longer sleep duration predicted a stronger cortisol response, because previous studies linked shorter sleep duration with higher cortisol levels,"  Mrug noted.

The study was published online in the journal Physiology and Behaviour.

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