Teenagers who use mobile devices for more than four hours a day sleep at least one hour less than normal. They fall asleep half an hour later and wake up half an hour earlier, compared with those who spend one hour or less in front of a screen every day, according to a study carried out by Neuroscience and Public Health institutes in the Netherlands, which was presented recently at the Annual Meeting of the European Endocrinology Society.

When these young people have less exposure to the 'blue light' which is emitted by mobile phones, tablets and computers, the symptoms of sleep disorder, which include fatigue, a lack of concentration and bad moods, disappear in just one week. The improvement is especially noticeable when use of the devices is restricted at night, because that is when more melatonin, which influences time and quality of sleep, is produced.

In Spain children spend an average of five hours a day in front of some type of screen and more at weekends, according to a study by the Association for Research into Means of Communication. The situation in neighbouring countries is similar.

"Teenagers are spending more and more time on devices with screens and sleep disturbance is becoming more common," says Dirk Jan Stenvers, a doctor at the Endocrinology and Metabolism department at the University Medical Centre of Amsterdam, who presented a study into sleep problems in adolescents in Lyon recently.

"We have shown that it can be easily reversed by minimising use of screens and exposure to 'blue light' at night. According to our data, it is probable that these disorders are caused at least in part by the blue light of these screens."

The study by the Dutch university concluded that, as well as keeping youngsters away from the screens near bedtime, blocking out this 'blue light' with special glasses also proved effective. With these two measures the patients were able to have a refreshing sleep, without waking up early, in just one week, says Stenvers.

"In the long term we know that lack of sleep is associated with a greater risk of obesity, diabetes and cardiac illness. If we can introduce simple measures now, we can avoid greater health problems in forthcoming years," he says.

However, once an adolescent becomes quite dependent on using the screen other variables come into play, such as anxiety or the uncontrollable need to be connected, a loss of interest in other activities and a denial of the problem manifesting itself in defensive behavour, say experts from the National Cybersecurity Institute.

Limiting use of the device is not going to be an easy task. Children, even the youngest ones, develop a tolerance to sitting at their screens, as with any addictive substance, and may suffer from withdrawal symptoms a well as irritability if use is restricted. There is also another factor: social approval from other users with whom the child interacts. Meanwhile the pathologies associated with excessive use of technological devices continue to increase.

Child obesity

Two other studies have related excessive use of mobile devices with health problems at increasingly earlier ages. One of them attributes the greater incidence of child obesity to this.

"We are bringing up a generation of sedentary children," warns Luis Blesa, president of the Valencian Pediatric Society. "Children are spending more and more time in front of a screen. That, combined with an unhealthy diet, can increase the number of obese children".

Another study attributes excessive use of screens to a progressive loss of vision. Children aged between seven and 12 have more myopia than university students a generation ago. Specifically, half a dioptre more, which can reach as much as six dioptres, according to Fabio Delgado, the optical director of Cottet.

"The principal cause of myopia is the growth of the ocular globe and that doesn't only occur for genetic reasons," says Enrique Jiménez, an optician at the San Joan de Déu hospital. "There are other very important reasons, such as the excessive number of hours the children spend looking at devices such as computers, tablets and mobile phones".

He recommends the same solution: reduce exposure time to these devices. "We need to restrict the time children spend on close-up tasks, especially with electronic devices, and make sure they spend more time in open spaces," he says.