Recently, a scientist from Osaka Metropolitan University learned that closed-eye brightness is stronger than previously reported, even indoors during a night’s sleep. Studies have shown that changes in light exposure during the nighttime or during sleep have been reported to influence circadian rhythms and sleep quality. Understanding the light transmittance of the eyelids and perceived closed-eye brightness is essential to properly describe the lighting environment during sleep. (Osaka Metropolitan University. "Casting light on, and through, your eyelids." ScienceDaily. ScienceDaily, 19 December 2022. <www.sciencedaily.com/releases/2022/12/221219094858.htm>.)
Professor Hideki Sakai, from the Graduate School of Human Life and Ecology at Osaka Metropolitan University, applied a new method to measure the light transmittance of the eyelids when the eyes are shut. A lighting device was used to increase or decrease facial illuminance, and closed-eye light transmittance was measured by having the 33 participants make adjustments to match the levels of brightness they perceived with their eyes closed and with their eyes open. Monochromatic red, yellow, green, and blue LEDs and a white LED were used as light sources. The results showed that eyelid transmittance values were up to 10 times higher than those reported in the past. The color of the light also made a difference, with red light perceived as brighter and blue light perceived as darker. Additionally, Professor Sakai noted significant differences between individual participants. For some, their perception of brightness remained almost unchanged between the open- and closed-eye conditions.
Light transmission through the eyelids has an impact on healthy sleep. When we sleep in a room with only light enough to see but not to read, the sympathetic arm of the autonomic nervous system responsible for the body’s fight or flight response remains active, disrupting sleep patterns. Reportedly up to 40 percent of us sleep with a bedside lamp on or with a light on in the bedroom, or keep the television on, not to mention a partner who may be scrolling on a smart phone late into the night. Sleep deprivation and sleep disorders are detrimental to health and interfere with daily life.
When it comes to light exposure that can be detrimental to the eye and overall health, eyelids don’t offer the protection we may have thought they did. So keep the lights off and the sunglasses on.