Although children require about nine hours of sleep throughout adolescence, older kids are naturally inclined to stay up later than younger ones, the study confirmed.

"This is one of the few studies that has tracked sleep behaviour and circadian rhythms over the course of up to two-and-a-half years in the same adolescents," said lead author Stephanie Crowley, assistant professor at the Rush University Medical Center, Chicago in the US.

The negative effects of inadequate sleep among teenagers include poor academic performance, mood disturbances, depression, obesity, and even drowsy driving accidents among older teenagers.

The researchers found that weekday sleep time continued to shrink with age for the 56 volunteers, aged 15 or 16, who participated in the research.

The typical kid in the study moved to a later sleep/wake cycle with ageing, except during the week, when later waking was not possible due to early schooling hours.

The study appeared in the journal PLoS ONE.

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