These days many people are glued to their phones, with digits on screens the last thing we see at night and the first thing we look at in the morning. But if these habits seem familiar to you, you may be damaging your ability to get a good night’s rest. The more you are glued to your smartphone or tablet, the poorer your quality of sleep, a new study has warned. The devices, which emit blue light, have become more common in recent years, and so has insomnia and other sleep disorders. But questions remained over their impact on sleep, University of California San Francisco researchers said. The new study, of US adults, found younger users were on the devices for longer and reported having a more disturbed night’s sleep. Concerns have been mounting as poor sleep affects schooling and work performance and is linked to depression, and is a risk factor for obesity, cardiovascular disease, stroke, and death. Over two thirds of adults sleep with their phones by their bed. Studies also showed hospital patients who read eReaders took longer to fall asleep and had reduced quality of sleep than those who read a traditional book, and those who watch the screens late at night before bed were most affected. Scientists argued this could be due to what they are viewing such as Facebook updates were more stimulating or blue light could be disrupting the body clock. The 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 US adults participating in the Health eHeart Study. “Smartphones are increasingly integrated into everyday life,” said co-author Matthew Christensen, from the University’s Division of Cardiology. “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 native environments have not previously been available.” Participants installed a smartphone application which recorded their screen-time, defined as the number of minutes in each hour that the screen was turned on, over a 30-day period. They also recorded their sleeping hours and sleep quality. It found that each participant had an average of 38.4 hours over this period, with smartphones being activated on average for 3.7 minutes in each hour during the day. He added, “Longer average screen-time was associated with shorter duration of sleep and reduced sleep efficiency. Given that screen-time after self-reported sleeping hours and near an individual’s bedtime was associated with reduced sleep efficiency and greater sleep onset latency, the relationship between overall smartphone use and sleep may be driven by exposure near bedtime.” But it does not necessarily mean people have to stop looking at their phones. There are apps that strip this light out of devices. For instance, Apple recently added a ‘night setting’ to its software so iPhone and iPad users can avoid blue light at night. The setting, Night Shift, changes the colour of the screen to a warmer, yellow-and-reddish hue at night, to apparently make it easier to sleep after using the device. F lux and Twilight are other pieces of software that alter screen light colour.