The study, conducted over 15 years, won the team the second prize in the State Science and Technology Awards last month.

Minors should sleep at least nine hours, with the youngest children needing longer, according to Dr Zhang Bin , a specialist researching insomnia at Guangdong General Hospital.

Comparing children in China and Switzerland, Shen said the sleeping hours of children in Switzerland decreased steadily as they grew up, but mainland children saw their sleep time decrease abruptly at around six years old, when they entered primary school.

'We think Chinese children's lack of sleep is related with their schooling activity,' he said. 'Our study found that there are two reasons leading to children having less sleep: too much homework and having to get up early in order to arrive at schools.'

Their study has also found that the less sleep children have, the more likely they will become fat. Shen said obesity was caused by multiple factors, the top three of which were: diet, children's body growth and sleep. Sleep has a bigger effect than physical exercise, the study says.

Children who get less sleep perform worse than their well-rested peers in terms of memory, concentration, the ability to learn languages and mathematics, communication with classmates and teachers, and academic scores, Shen's study showed.

Dr Yao Peifen, from Shanghai Mental Health Centre, said most Shanghai middle school pupils go to bed after 11pm and get up before 7am.

She said high levels of pressure to perform academically made many pupils depressed. Her hospital's juvenile department had received 20,000 patients last year, most of whom were suffering from depression. 'Children have a lot of homework to finish and the homework is often difficult,' she said.

Based on the team's study, Shen proposed that the Shanghai government postpone the starting time of primary and middle schools from 7.15am to 8.15am, which it did in 2007. The study also found that the average sleep hours of children in the city had increased from 9.03 hours per day in 2005 to 9.24 hours.

The public health school at Guangzhou's Sun Yat-sen University investigated 1,000 children in Zhongshan and found 365 of them had sleep problems, the Nanfang Daily reports.

Adults are also not getting enough sleep, Zhang says. He surveyed 10,000 people in Guangzhou in 2009 and 30 per cent said they slept less on workdays and 20 per cent more on weekends. He said city dwellers faced stress from their jobs and could stay up enjoying entertainment options at night.

World Sleep Day was initiated by the World Association of Sleep Medicine in 2008. This year's theme is 'Breathe easily, sleep well', with a focus on boosting awareness of obstructive sleep apnea - a potentially serious disorder in which breathing repeatedly stops and starts during sleep.

But mainland authorities regard insomnia as the bigger problem, said Dr Xu Liang, from the Shanghai Hospital of Traditional Chinese Medicine (TCM), and have opted instead for the theme 'Healthy sleep, happy China'.

Xu says he has practised TCM to treat sleep troubles for two decades. More than 70 per cent of his patients - more than half of whom are younger than 50 - suffer from insomnia. A few of them can only sleep for three hours a day.

'They are too worried about job competition and rising living costs,' Xu said.

He said a key reason for increasing rates of insomnia was that people now stay up quite late, with old people fond of watching television and young people spending their time in front of computers.

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Hours Briton Tony Wright stayed awake in 2007, in an attempt to break the world record for sleep deprivation, which is 276 hours