Nine out of 10 children fail to get the recommended eight hours of sleep a night, according to a recent study – a fact that can have serious long-term implications for their mental and physical health. Now researchers have found a way to get teenagers to gain an extra 50 minutes of sleep a night - and all it took was a flashing light and some positive thoughts.

Insufficient sleep in teenagers is increasingly being recognised as a public health concern, linked to obesity, memory problems, concentration issues, poorer academic performance and mental health issues. One study has shown that teenagers with no set bedtime who go to sleep late are 24 per cent more likely to suffer from depression.

The battle to get teenagers to go to bed earlier is an ongoing one due to the physiological changes during puberty. Our circadian rhythm controls when we naturally want to wake and sleep, and this rhythm shifts with age. While young children tend to wake up earlier, as we hit puberty our bodies want to go to bed later and wake up later. This continues until around the age of 20, after which our bodies' waking times gradually get earlier, returning to that of our 10-year-old self once we hit 55.

While entering puberty is a challenging time, when it comes to sleep, teenagers are fighting against what they want to do versus what society requires them to do. Early school start times require teenagers to wake up earlier than they might do naturally. To cope with this, teenagers need to go to sleep earlier than they naturally want to in order to get enough sleep for their mental and physical health. The problem is that just sending teenagers to bed early doesn't mean that they will fall asleep – what's needed is a way to help reset their biological clock so they feel sleepy earlier.

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Previous research into jet lag has shown that exposure to short flashes of light can trick the brain into adjusting to a new time zone. It is thought that the neurons that project from the retina in the eye to the suprachiasmatic nucleus in the brain – the system that resets the circadian rhythm - are more active when exposed to light flashes than continuous light.

Using this knowledge, researchers in California experimented with flashing light in the bedrooms of teenagers aged 14 to18, at a level that was enough to reset their circadian rhythm but not so high as to wake them. They found that three milliseconds of light flashed every 20 seconds during the final three hours of the teenagers' sleep cycle was the most effective dosage.

Although the light flashes helped them to wake earlier, the light therapy alone failed to convince the teenagers to go to bed earlier – as a result, they were still sleep deprived.

Next, the researchers tried cognitive behavioural therapy to help convince the teenagers to go to bed earlier by finding out what motivated them. This included physical appearance, athletic performance and academic achievement. By focusing on achieving these outcomes through more sleep, the teenagers went to bed 15 minutes earlier than normal.

The real power came when both interventions were combined. The newly motivated teenagers with their light-shifted circadian rhythms went to sleep an average of 50 minutes earlier.

The research published in the journal JAMA Network Open shows that a beautifully simple combination of light flashes from a programmable light and some positive motivational thoughts could be a cheap and easy way to help our teenagers get enough sleep, and potentially realise some significant benefits to their health and wellbeing.