On school days, junior college student Wong Rui En, 18, doesn't go to bed till past midnight, but has to drag herself out of bed at 6.30am.

She ticks off the list of things she has to do. School ends at 4.30pm, and she has co-curricular activities or tuition till after 8pm, or even 10pm. Then she gets down to homework.

"Sure, I feel tired. But I've got used to it," she said with a shrug.

Rui En is a typical teen here.

Research is increasingly showing that such a truncated amount of sleep for children in her age group has a detrimental impact on mental and physical health.

A new study by the Duke-NUS Medical School found that teens who had insufficient sleep did worse cognitively than teens on the recommended full amount of sleep for their age group, even after just a week.

The researchers found that seven nights of shortened sleep resulted in "prominent deterioration" of sustained attention, working memory, alertness and foul mood.

"Despite the fact that most of our participants were from elite schools, they were not spared the adverse effects of sleep curtailment on their cognitive functions," said Dr June Lo, lead author of the study and a Duke-NUS senior research fellow.

Professor Michael Chee, senior author and director of the Centre for Cognitive Neuroscience at Duke-NUS, urged parents, schools and students themselves to use time more effectively to prioritise sleep. He said at one school, they found that more than half the students slept less than six hours.

The increasing use of electronic devices such as tablets and smartphones is one culprit.

Prof Chee said: "Although the teens know that sleep is important, they put a low priority on it. "

Dr Toh Song Tar, director of the sleep disorders unit at Singapore General Hospital (SGH), also noted the excessive screen time spent on computer games and electronic devices. "Sleep may be interrupted by messages coming in the middle of the night if the adolescents want to be connected all the time," he said.

Teens conceded that they spent a lot of screen time just before sleep.

"I do surf the Internet after finishing my homework, and watch videos, but they help me to de-stress, even though I feel I need to sleep more," said Rui En.

She also reasoned that her hectic schedule would last just two years.

A solution that has been proposed by Prof Chee and others is to shift school schedules to suit the circadian rhythms of youth, as well as their heavier work schedules.

"The key to the future is also a later school starting time," said Prof Chee.

As one grows into late adolescence, the body clock prefers a later sleep time, he said.

But our schools start quite early compared with those in other countries. "We should consider changing the start time. This has also been raised by parents and educators here," he added.

He also called for the Health Promotion Board to advocate sleep in its promotional materials.

People should prioritise sleep to be as important as eating well.

Insufficient sleep affects general health and leads the body to overeat, instead of good habits such as exercise, said Prof Chee.

"If you don't sleep enough, your body will be too tired to exercise, and your brain will tell you to keep eating instead," he said.

According to the National Sleep Foundation in the United States, the recommended hours of sleep are between eight and 10 hours for teenagers, and seven and nine hours for adults aged between 18 and 64 years. Adults who are older than 64 years should be sleeping between seven and eight hours

Dr Toh said the SGH sleep disorders unit has seen an increasing number of patients with insufficient sleep due to sleep disorders or insomnia.

"Lack of sleep limits a person's ability to learn new things and impacts negatively on short- and long-term memory. Chronic sleep deprivation can interfere with decision-making abilities," he noted.

Sleep deprivation also affects the immune system and other functions, he added.

Dr Adrian Saurajen, an ear, nose and throat surgeon at Mount Elizabeth Hospital who specialises in sleep problems, elaborated that sleep deprivation affects different aspects of cognition differently.

For instance, lack of sleep leads to reduced response speed, particularly for alertness, attention and vigilance. It also affects creative function. "But planning tasks and reasoning are affected less," he said.

There is also a slew of medical problems associated with insufficient sleep, he said. It includes diabetes, obesity and heart disease. Life expectancy also gets cut short.

Many of his patients, he noted, not only have insufficient sleep, but also have poor sleep quality, such as restless sleep or difficulty falling asleep.

Common reasons for poor quality of sleep include lifestyle issues such as alcohol and caffeine consumption, smoking and irregular sleeping patterns. It could also be due to psychological problems or insomnia, he said.

He has a tip for anyone wanting more sleep. "I ask patients to let their bodies wake them up. I am not a big fan of using alarm clocks to wake up. We have a natural sleep cycle and we should always try and go to bed and wake up at the same time," he said.

Generally, the younger the children, the longer the hours of sleep they need. But at a pinch, adolescents should get at least seven hours of sleep a night, said Prof Chee.

Dr Toh added that parents should take a more active role in restricting device use prior to sleep and getting adolescents to put away their smartphones when it is time to sleep.