Our body starts to produce the sleep hormone melatonin - which helps people to fall asleep - as it gets darker in the evening, researchers said. Wavelengths of light at the blue-green end of the visible spectrum can disrupt the system.

Paul Gringras, from Evelina Children's Hospital in London, analysed the light emitted by devices for the study, that found a clear trend for new devices to be bigger, brighter, have higher levels of contrast and emit more blue light.

"That is great for use in the day, but awful for use at night. There is converging data to say if you are in front of one of these devices at night-time it could prevent you falling asleep by an extra hour," Gringras told media.

Gringras said that smartphones should have settings to filter out the blue light that delays the body clock and keeps people awake later into the evening.

He said some sleep-aware apps had already been designed to reduce blue-green light emissions, and that a bedtime mode could automatically filter out the blue as software such as 'f.lux' already does.

The study was published in the journal Frontiers in Public Health.

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