Blue light (emitted from PC and smartphone screens) suppresses the secretion of melatonin (the sleep hormone). Therefore our quality of sleep is sacrificed because we "sleep with our phone".
Screen color regulating apps can help. Depending on the time of day they decide which colors to display.
F.lux is a good PC program for that.
Not looking at screens 2 hours prior to going to bed would also help, but the study shows we spend more time on our phones especially before wanting to sleep.