The effect of narrowband 500 nm light on daytime sleep in humans

Naps frequently take place during the daytime under some ambient light. People are commonly advised to wear eyeshades, or use black-out curtains while sleeping, as light is thought to inhibit sleep. Little is known, however, about how light during daytime sleep may affect the quality or architecture of that sleep. The present within-subjects design administered green narrowband light via light masks to 17 young adults (23.2 ± 4.7 years) during four 90-minute afternoon naps. Subjects were exposed to each of four light conditions that approximate the intensity of 1) physiological darkness (~ 0 lx), 2) moonlight (~ 1 lx), 3) typical indoor lighting (~ 80 lx) and 4) indirect outdoor light (~ 6400 lx). All subjects were able to sleep in all lighting conditions, with no differences in sleep quality or architecture. Power analysis revealed sufficient power to detect meaningful differences. Sleep inertia measured upon waking showed a general effect of the nap, independent of condition. Although light has various alerting effects at night, 500 nm LED light presented via light mask does not appear to inhibit daytime sleep. The finding that this light had no effect on the ability of individuals to fall asleep or stay asleep during an afternoon nap may inform decisions regarding the use of the nap as a facilitator of schedule adjustment, and challenges the assumption of light as a barrier to napping.

Research highlights
► Narrowband green light was administered via a light mask during daytime sleep.
► Four lighting conditions were tested, ranging in intensity from 0 to 6400 lx.
► Sleep variables measured during napping correlated with duration of prior sleep.
► All sleep measures were independent of light intensity.