School closures earlier in the pandemic harmed children's quality of sleep even though many spent longer in bed, Hong Kong researchers said on Monday.

A team from the University of Hong Kong looked at the sleep patterns of 759 primary school students and 1,140 secondary school students between September 2019 and July 2021.

During part of that time, classes were suspended due to Covid outbreaks and on average students slept an hour longer than normal, the researchers said.

However, the researchers said students also went to bed significantly later than usual and this could have harmed their health.

Professor Patrick Yip from the university’s department of paediatrics and adolescent medicine said delaying bedtime can reduce the amount of melatonin and growth hormones produced, and the spinal fluid in the brain will also decrease.

“[Sleep delay] has a huge impact. Longer sleep cannot make up for the bedtime delay,” he said.

“If the sky goes completely dark and you don’t go to sleep, your body will not adjust your hormone properly and cannot produce enough melatonin. It poses a huge [negative] impact on sleep quality, health and studies.”

The researchers also noted that the amount of physical activity the students did went down when the schools were closed, adding that exercising more can also improve sleep quality.

Yip said the World Health Organisation recommends adolescents aged five to 17 take part in aerobic exercise for at least one hour a day, but their study found primary school students were only doing 30 minutes a day on average, while secondary school students did only half that.

He urged children to do an extra 15 minutes of exercise per day to improve both their physical and mental health.

“[An additional] 15 minutes, of course, is just a very short period of time. But we believe 15 minutes actually is a good start. It’s a minimum requirement,“ he said.

Meanwhile, the chief executive of the Inspiring Hong Kong Foundation, Judy Kong, called on the government not to shut schools and sports facilities again, even if there's another wave of Covid infections.

“We might not encourage them to close the schools again, that could be the very last option if there’s any other wave of the pandemic,” Kong said.

“If the vaccination rate among children is high, we also encourage that during the next wave, we should still [keep] the sports facilities open, instead of closing all of them. That would be also helpful for parents, especially those from low-income backgrounds, to find space to do sports in the community outdoors.”