Is an early night the key to better test results?

Young people who go to bed earlier and sleep for longer have greater brain function and could perform better in tests, a study suggests.

While a good night's sleep was important, "surprisingly little" was known about sleep in adolescence, said Prof Barbara Sahakian of the University of Cambridge.

Researchers from Cambridge, and Fudan University in Shanghai, found that those who slept for the shortest time had poorer brain functions - and better sleep could benefit memory and heart rate.

Prof Sahakian, of Cambridge's department of psychiatry, said that even with 15 minutes' more sleep, ".. we could still see differences in brain structure and activity and in how well they did at tasks."

"This drives home to us just how important it is to have a good night's sleep at this important time in life," she said.

Researchers tracked the sleep of 3,222 children aged between 11 and 12 using FitBit devices.

They compared the results against two groups of 13 to 14-year-olds, comprising 1,190 youngsters, and divided the children into three categories based on sleep times.

The first slept, on average, for seven hours and 10 minutes, while the second group slept for seven hours and 21 minutes, and the third slept for seven hours and 25 minutes.

Researchers found the group that slept the longest had the largest brain volume, best brain function and lower heart rates, usually a sign of better health.

The first group had the smallest volume and poorest brain function.

Researchers also found no difference in school achievements between the three groups, but when given cognitive tests in reading, vocabulary and problem-solving, the group that slept the longest performed the best.

The study, published in Cell Reports, also used data from the Adolescent Brain Cognitive Development Study.

Dr Qing Ma, of Fudan University, said: "Although our study can't answer conclusively whether young people have better brain function and perform better at tests because they sleep better, there are a number of studies that would support this idea.

"For example, research has shown the benefits of sleep on memory, especially on memory consolidation, which is important for learning."

Senior author Dr Wei Cheng added: "Given the importance of sleep, we now need to look at why some children go to bed later and sleep less than others.

"Is it because of playing videogames or smartphones, for example, or is it just that their body clocks do not tell them it's time to sleep until later?"