The light from these devices can throw off your body's natural rhthyms and may make you vulnerable to diseases like cancer and diabetes. There is a lot of light pollution now than there ever has been. We are disrupting our circadian rhythms. Light exposure at the wrong time of day reduces the amount of the hormone melatonin in animals' bodies. Studies have show that melatonin helps protect against cancerous tumors. The glow from screens tricks our bodies into thinking it's day. People that use them a lot at night produce less melatonin and could be putting themselves at risk.
The companies that produce these devices know this and should warn their consumers, who don't generally know this.