A study found that using glowing e-book readers and other electronic light before bedtime may alter circadian rhythms and have a negative impact on sleep and alertness. Darker light at night is a cue to circadian rhythms and can promote the production of melatonin, a hormone that regulates sleep. To investigate this effect of bright artificial light from devices such as light-emitting e-readers on sleep, the quality of sleep obtained after subjects used an e-book reader before going to bed was compared to the quality of sleep obtained after reading a printed book before going to bed. During a 14-day study, 12 subjects used a lighted e-book reader to read in dim light for four hours before going to bed for five days, and they also read a printed book before going to bed for five days under similar conditions. When subjects read with an e-book reader instead of reading a printed book, subjects had less nighttime sleepiness, took longer to fall asleep, and reported reduced morning wakefulness. When using a lighted e-book reader, subjects exhibited suppressed nighttime melatonin levels and altered sleep/wake circadian rhythms, which affected the time to fall asleep and wakefulness the next day. These results suggest that the widespread use of e-book readers and other light-emitting electronic devices may affect alertness and health, the group of authors said.