Unrestricted evening use of light-emitting tablet computers delays self-selected bedtime and disrupts circadian timing and alertness

Recent studies have demonstrated that use of light‐emitting (LE) electronic devices in the evening close to bedtime negatively affects physiology and behavior, especially that of sleep and circadian rhythms (Bonnefond et al. 2006; Van den Bulck 2007; Cajochen et al. 2011; Munezawa et al. 2011; Arora et al. 2013, 2014; Czeisler 2013; Foley et al. 2013; Wood et al. 2013; Gamble et al. 2014; Heath et al. 2014; Chang et al. 2015; Carter et al. 2016; Czeisler and Shanahan 2016). Rapid technological advances and market pressures have led to near ubiquitous use of LE‐devices (e.g., desktop, laptop, and tablet computers, cell/smart phones, televisions, and video games) in the modern home and workplace; and such devices are now integral to many daily functions including communication, commerce, recreation, and access to news and information. Greater utility, availability, and portability of LE‐devices have additionally led to widespread LE‐device use in the evening and incorporation into the bedtime routine. A recent National Sleep Foundation survey found that 90% of Americans reported use of LE‐devices within an hour of their bedtimes, and greater use was associated with worse sleep outcomes at all ages (Gradisar et al. 2013). That and other studies have shown that adolescents and young adults are particularly likely to use LE‐devices before bedtime, often multiple LE‐devices at the same time, and the amount of LE‐device use in the evening is associated with multiple negative outcomes including delayed bedtimes, longer sleep latencies, sleep interruption from the LE‐devices during the night, shorter sleep durations, increased daytime sleepiness, and even obesity (Van den Bulck 2007; Munezawa et al. 2011; Arora et al. 2013, 2014; Foley et al. 2013; Gradisar et al. 2013; Gamble et al. 2014; Pourzanjani et al. 2015; Carter et al. 2016; Czeisler and Shanahan 2016). Light exposure per se is thought to cause many of the negative effects from evening use of LE‐devices. Retinal photoreceptors transmit light information to the master circadian clock in the hypothalamus, the suprachiasmatic nucleus (SCN), which regulates secretion of the pineal hormone melatonin together with rhythms in physiology and behavior including sleep, metabolism, immunity, alertness, and performance (Goel et al. 2013; Scheiermann et al. 2013; Qian and Scheer 2016). In the evening hours around bedtime, light exposure suppresses melatonin secretion and causes a phase delay shift in circadian rhythm timing such that melatonin secretion is reset to begin at a later time on subsequent nights (an effect similar to jet lag from extending daylight exposure later due to westward travel) (Khalsa et al. 2003; St Hilaire et al. 2012). This in turn can induce misalignment between the timing of the circadian rhythm of sleep propensity and the timing of sleep, reducing the duration, and quality of sleep (Czeisler et al. 1980; Dijk and Czeisler 1994). Misalignment can also cause delays in bedtime which, especially in children and adolescents, are associated with worse outcomes including behavioral risk factors and mental health problems such as depression (Gangwisch et al. 2010; Lemola et al. 2015; McGlinchey and Harvey 2015). There are also acute effects of light exposure that increase physiological and subjective levels of alertness (Lockley et al. 2006; Cajochen 2007; Rahman et al. 2014), which may impact when an individual feels tired and chooses to go to sleep when they are exposed to light in the evening. Furthermore, the effects of light on sleep, alertness, and circadian physiology are even greater from light exposures that are brighter, longer in duration, timed later in the evening, and contain short‐wavelength light (Lewy et al. 1980; Zeitzer et al. 2000; Brainard et al. 2001; Thapan et al. 2001; Khalsa et al. 2003; Lockley et al. 2006; Cajochen 2007; Santhi et al. 2012; St Hilaire et al. 2012; Lucas et al. 2014; Rahman et al. 2014). Because LE‐devices are often used for extended periods close to bedtime and their screens are typically illuminated by light‐emitting diodes (LED) that are rich in short‐wavelength light (Chang et al. 2015; Gringras et al. 2015) to which the human circadian system is particularly sensitive (Brainard et al. 2001; Thapan et al. 2001; Lockley et al. 2003; Ruger et al. 2013), their use is likely to increase alertness and impact sleep. Our group reported findings from a laboratory‐controlled, within‐subject, counter‐balanced, and randomized study in which the effects of light exposure from five consecutive evenings of reading on a LE‐tablet computer were compared to five consecutive evenings reading from printed books prior to a strictly imposed 10 pm bedtime, followed by an 8‐h scheduled sleep episode in total darkness (Chang et al. 2015). Compared to the evenings with printed books, the LE‐tablet reading condition caused significant melatonin suppression, circadian phase delay shifts, longer sleep latencies, reduced rapid eye movement (REM) sleep, increased alertness prior to bedtime, and lower next‐morning alertness. Those findings provided direct evidence that the light exposure from a tablet computer negatively affected sleep, circadian rhythms, and alertness under controlled laboratory conditions. In that study, the duration of evening reading sessions was set to four hours, bedtimes were fixed at 22:00, and in the LE‐tablet condition the device was placed at a fixed distance from participants and activities were restricted to reading electronic books (eBooks). Other studies that tested the effects of LE‐devices have similarly controlled aspects of the duration, timing, illuminance, and screen content of light exposures (Cajochen et al. 2011; Wood et al. 2013; Heath et al. 2014; van der Lely et al. 2015; Gronli et al. 2016). Now that the effects of LE‐devices on sleep, circadian rhythms, and alertness have been established even when bedtimes are held at a fixed time under controlled laboratory conditions, it is critical to evaluate the impact of LE‐devices on self‐selected bedtimes, sleep, alertness, and circadian rhythms in a randomized trial under controlled laboratory conditions. Increased evening alertness, melatonin suppression, and delayed circadian rhythm timing are likely key factors contributing to epidemiological findings showing associations between evening LE‐device use and later bedtimes (Foley et al. 2013; Gamble et al. 2014; Hysing et al. 2015). Additionally, such devices have numerous capabilities (e.g., internet, email, social media, games, videos, calendar, live streaming). Personal engagement with the diverse available activities may contribute to the selection of later bedtimes, thereby further prolonging evening light exposure from the device, and potentially leading to similar or even greater effects on sleep and circadian rhythms as those already observed under conditions with fixed bedtimes and light exposures. Therefore, in a follow‐up investigation to Chang et al. (Chang et al. 2015) we utilized a similar study design under controlled laboratory conditions, while allowing participants to self‐select their nightly bedtimes and LE‐tablet activities. We aimed to test whether under these unrestricted conditions, evening use of LE‐tablets compared to reading printed materials would affect self‐selection of bedtimes, and influence melatonin secretion, sleep, circadian timing, and alertness.