An American study shows that those who routinely look at their smartphones and tablets before bed may be wrecking their sleep.
The blue light from screens as we send texts, check emails and watch streamed television is thought to make it harder to drift off.
Concerns have been mounting over our use of handheld devices at night, as poor sleep affects work performance and schooling and is linked to depression. It also increases the risk of obesity, cardiovascular disease and stroke.
An investigation into the effects of gadgets found younger users were on the devices for longer and reported their sleep was worse. Those who watch the screens late at night before bed were the worst affected.
Scientists at the University of California San Francisco argued this could be due to their brains being stimulated by what they were viewing, such as emotional Facebook updates. The blue light from the screens could also disrupt the body clock.
Over two thirds of adults sleep with their phones by their bed. Previous studies show that hospital patients who used eReaders took longer to fall asleep and had reduced sleep quality than those who read a book.
The latest study, published in the journal PLOS ONE, tested the hypothesis that increased screen-time may be associated with poor sleep by analysing data from 653 adults participating in the Health eHeart Study.
Participants installed a smartphone app which recorded their screen-time, defined as the number of minutes in each hour that the screen was turned on, over a month.
They also recorded their sleeping hours and rated their quality of sleep.
The study found that each participant had an average of 38.4 hours of smartphone use over this period, with phones being activated on average for 3.7 minutes in each hour during the day. Heaviest users were the young, black people and other minority ethnic groups.
‘Smartphones are increasingly integrated into everyday life,’ said Matthew Christensen, from the Division of Cardiology at the University of California San Francisco Medical Centre.
‘At the same time, the prevalence of insomnia and sleep deprivation have risen. Poor sleep – too little or too much, and poor quality – has been shown to be a risk factor for obesity, diabetes, cardiovascular disease, depression, and overall mortality.
‘Light in the blue spectrum, such as light produced from a smartphone, can suppress production of melatonin, leading to decreased drowsiness, difficulty initiating sleep, and non-restorative sleep.
‘In addition, engrossing activities during smartphone use may result in stimulation that is counter-productive to sleep preparation.
‘Limiting the use of TV and computers near bedtime is commonly recommended as an important part of good sleep hygiene, but direct measurements of screen-time in home environments have not previously been available.’
The researchers said they could not exclude the possibility that being unable to sleep may be leading people to look at their phones.