Studies have shown that the use of electronic devices 4 hours before bedtime affects the secretion of melatonin and causes insomnia Prolonged use of portable light-emitting devices (laptops, tablets, smartphones) before bedtime can have a negative impact on melatonin secretion, circadian rhythms, and sleep. One study compared the effects of reading an electronic book illuminated by a light-emitting device (LE-ebook) versus a printed book (by reflected light) for four hours prior to bedtime for five consecutive nights . Subjects in the LE-ebook group had suppressed melatonin concentrations in the early part of the night, a delayed endogenous circadian melatonin phase, felt less sleepy before bed, took longer to fall asleep, and reported feeling sleepier the following morning. These observations suggest that evening use of light-emitting devices may contribute to phase delays in the circadian clock and difficulty initiating sleep.