The American Academy of Sleep Medicine recommends that teens get at least nine hours of sleep each night. (iStock)

Teens who sleep less than eight hours on weeknights tend to eat more fatty foods than those who doze longer, researchers have found.

The sleep study involved 240 Cleveland teens ages 16 to 19.

Teens who slept less than eight hours on weeknights consumed 2.2 percentage points more calories from fats and 3.0 percentage points fewer calories from carbohydrates than teens who slept eight hours or more, after taking factors like age and sex into account, researchers said in Wednesday's issue of the journal Sleep.

The changes in diet patterns offer insight into why less sleep has been linked with obesity in previous studies, said the study's senior author, Dr. Susan Redline, a professor of medicine at Brigham and Women's Hospital in Boston.

For each one-hour increase in sleep, the odds of consuming a high number of calories from snacks also decreased by an average of 21 per cent, the researchers found.

Trained staff conducted two dietary intake interviews to collect details about food and portion sizes, as well as the timing, location, type and preparation of meals and snacks.

In the study, average weeknight sleep duration was calculated using at least three nights of data.

The average weeknight sleep duration was 7.55 hours. About a third of the teens slept for an average of eight hours or more.

The American Academy of Sleep Medicine recommends that teens get at least nine hours of sleep each night to feel alert and well rested during the day.

The study cannot show whether there is a cause-and-effect relationship, but Redline said physiology studies have revealed several ways in which sleep loss may promote weight gain.

For example, getting less sleep is thought to change one's metabolic rate and affect the production of leptin and ghrelin, two hormones that regulate appetite.

Redline said sleep might be the missing link in the fight against obesity — which has focused solely on diet and exercise.

Less sleep increases risk of mental health problems

A second study appearing in the same issue of the journal suggested young people who get less than five hours of sleep may triple their risk of developing a mental illness compared with those who sleep for eight or nine hours.

The George Institute for Global Health at the University of Sydney surveyed almost 20,000 Australians between the ages of 17 and 24 for the research.

Over the past few decades, kids have more gadgets in their bedrooms than before, and they've generally been sleeping fewer hours but starting school at the same time, said the study's lead author, Professor Nick Glozier of the institute.

It is important to prevent mental health problems in young people, Glozier said.

"It's those chronic mental health problems when you're an adolescent or you're a young adult that lead on to the more important adult forms of the disorders, like major depressive disorder or bipolar disorder," he said.

"So, if we can do something around that group of people when they're beginning to become chronic, or preventing those chronic, persistent problems, then we may have a really good target for an early intervention."

Sleep disturbance might affect the timing of biological rhythms that could be measured to identify people at risk of developing mental health problems, said Dr. Patrick McGorry, a professor of youth mental health at the University of Melbourne.

With files from the Australian Broadcasting Corp.