Scientific research suggests that exposure to blue light before sleep distorts your natural (circadian) rhythm resulting in an inability to fall asleep. See, Melanopsin, the light receptor in your eyes, is very sensitive to a narrow band of blue light in the 460-480nm range. It's not a shortcoming of nature, it's just part of its job.
Anyway, this particular range happens to be emitted generously by your smartphone or tablet's display, and it also happens to suppress Melatonin production, which ruins your sleep-wake cycle. The effect can result from you falling asleep a full hour later than you'd like, to full blown insomnia. Yikes! If you have the habit of bringing your phone or tab to your bed, you're better off installing one of those apps. It's certainly easier than procuring a new set of eyes or having to doze off on sleep aids, we reckon.