Sleeping with the light on not only causes shallow sleep and frequent arousals but also has a persistent effect on brain oscillations, especially those implicated in sleep depth and stability, according to a study published in Sleep Medicine. Authors Jounhong Ryan Cho, Eun Yeon Joo, Dae Lim Koo, and Seung Bong Hong found that lights-on sleep was associated with increased stage 1 sleep, decreased slow-wave sleep, and increased arousal index.