Consumer electronic devices play an important role in modern society. Technological advancements continually improve their utility and portability, making possible the near-constant use of electronic devices during waking hours. For most people, this includes the evening hours close to bedtime. Evening exposure to light-emitting (LE) devices can adversely affect circadian timing, sleep, and alertness, even when participants maintain a fixed 8-hour sleep episode in darkness and the duration of evening LE-device exposure is limited. Here, we tested the effects of evening LE-device use when participants were allowed to self-select their bedtimes, with wake times fixed as on work/school days. Nine healthy adults (3 women, 25.7 ± 3.0 years) participated in a randomized and counterbalanced study comparing five consecutive evenings of unrestricted LE-tablet computer use versus evenings reading from printed materials. On evenings when using LE-tablets, participants' self-selected bedtimes were on average half an hour later (22:03 ± 00:48 vs. 21:32 ± 00:27 h; P = 0.030), and they showed suppressed melatonin levels (54.17 ± 18.00 vs. 9.75 ± 22.75%; P < 0.001), delayed timing of melatonin secretion onset (20:23 ± 01:06 vs. 19:35 ± 00:59 h; P < 0.001), and later sleep onset (22:25 ± 00:54 vs. 21:54 ± 00:25 h; P = 0.041). When using LE-tablets, participants rated themselves as less sleepy in the evenings (P = 0.030) and less alert in the first hour after awakening on the following mornings (P < 0.001). These findings demonstrate that evening use of LE-tablets can induce delays in self-selected bedtimes, suppress melatonin secretion, and impair next-morning alertness, which may impact the health, performance, and safety of users.
