?Research shows that few extra minutes of sleep keep the child more alert and focused

The researchers write in the journal Sleep Medicine that few teenagers get the suggested eight to 10 hours of sleep per night and instead, they may try to make up sleep on weekends, which creates an irregular sleep pattern.

Teenagers in both Hong Kong and the US only get an average of about 7.5 hours of sleep on school days, said lead author Yun Kwok Wing of The Chinese University of Hong Kong.

Wing said, "Even a modest delay could lead to a number of positive outcomes", to Reuters by email, adding that sleep deprivation has been linked to emotional, behavioural and physical health issues for teens.

To explore how delaying school start times might affect teens, the study team collected data on 1,377 students from two Hong Kong high schools.

One of the schools normally started at 7:45 AM and let out at 3:40 PM and the other normally started at 7:55 AM and let out at 3:55 PM. For the experiment, the first school changed its start time to 8 AM, compensating for the delay by shaving 15 minutes from lunch hour and retaining the 3:40 PM release time. The other school made no changes in its schedule.

Before the study began, researchers evaluated 617 students at the school that agreed to delay its start time and 556 students at the comparison school. All the teenagers filled out a week-long sleep diary and a small group had their sleep monitored with sensors to validate the sleep diaries. Students also filled out questionnaires asking about their sleeping and waking patterns, their mood and their behaviours, including smoking, drinking and consuming caffeine. Five months later, the same measurements were performed. Based on this data, the study team found that students in the delayed-start school spent slightly longer in bed — about an extra 10 minutes in the mornings — compared to the control school’s students. Both groups went to bed slightly later as the study period went on and both continued to feel sleepy during the day and to oversleep on weekends.

But at the end of the study, the students in the delayed-start school had better overall mental health than the students in the comparison school. Their questionnaires and teacher reports indicated improvements in social behaviour and relationships with peers, and they had fewer emotional problems than the comparison students.

The later starting students also had fewer behavioural issues, better concentration in class, less dozing off and less tardiness, according to teachers. Nearly all teachers said they appreciated the later start time and that it improved their schedules and their own sleep, although some worried about possible changes in school dismissal time, class length and after-school activities. Three quarters of students at the intervention school said that the delayed start time was helpful for sleep, attention and getting to school on time, though a small group had issues with transportation and the shortened lunch period.