'Body clock' tells teenagers to stay up late

Tired teenagers are not to blame for being grumpy in the mornings - their bodies are telling them to stay up late, says an Auckland doctor.

Dr Simon Denny, who worked on a recent study of high school students' sleep patterns, said the trend in late bedtimes was not just social, but physiological.

"It's not just because they're hanging out with their mates until late at night ... it's a shift in their body clock which happens during their teenage years.

"This peaks in their early 20s, and then during adulthood there's a gradual decline back towards earlier bedtimes and waking times."

The shift was well-recognised phenomenon, said Dr Denny, but it wasn't clear what caused it.

"They are waking up later in the weekends, but going to bed much later." He said 17-year-olds were going to bed about 1am on weekends.

Dr Denny said the figures came from a 2001 survey which canvassed more than 9000 New Zealand students. It was a six-yearly project which would be repeated next year.

"[The research] has consequences in terms of the times we expect teenagers to be doing things in the morning," Dr Denny said.

Students who struggled with a lack of energy on school mornings were being told by their bodies they should still be asleep, he said.

Wellington High School introduced a 10.15am start for its older students in May by giving all Year 12 and 13 students a study period at the beginning of the day.

Principal Prue Kelly said the scheme would be reviewed before exams in October, but it had proved successful so far.

"Teachers are unanimous in saying that the students are much more responsive in the mornings.

"They're awake and they're asking questions."

Ms Kelly said she had witnessed the shifting sleep patterns in her students and her own children.

The study also showed that one in five teenagers wasn't getting enough sleep, and students who did more than five hours of part-time work in a week were more likely to be sleep-deprived.

- NZPA