Abstract
Background: Evening exposure to display light has been shown to prolong sleep latency and to have phase-shifting effects on circadian melatonin rhythms. All three photoreceptors (cones, rods and melanopsin-containing ganglion cells) contribute to the so-called non-image-forming (NIF) effects of light. To better understand isolated melanopsin-dependent NIF effects of evening display light, we generated light settings that matched cone excitation (metamers), but differed maximally in melanopsin contrast. Here, we aimed to observe melanopsin-dependent effects on sleep latency, melatonin concentration and melatonin onset as well as visual comfort in different luminance levels.
Methods: Seventy-two healthy, male participants completed a 2 -week study protocol. Volunteers were assigned to one of four groups, which differed in luminance levels (27 cd/m2-285 cd/m2). Within the four groups, each volunteer was exposed to a low melanopic (LM) and high melanopic (HM) condition. The metameric LM and HM differed in melanopic equivalent daylight illumination (mEDI): Group 1: mEDI 4 lx vs. mEDI 15 lx, Group 2: mEDI 9 lx vs. mEDI 33 lx, Group 3: mEDI 21 lx vs. mEDI 70 lx, Group 4: mEDI 48 lx vs. mEDI 146 lx. The two 17 h study protocols comprised 3.5 h of light exposure, starting 4 h before habitual bedtime, 8 h of bedtime and one hour of morning dim light. Poly-somnographically assessed sleep latency (time between lights off and first occurrence of sleep stage N2) was manually scored according to AASM. Before, during and after light exposure, salivary melatonin levels were measured in half-hourly intervals throughout scheduled wakefulness. To evaluate visual comfort, the volunteers filled in a Visual Comfort Questionnaire (e.g., acceptance of light situation, brightness, light color, and glare perception). Results: Sleep latency was significantly longer after HM than LM in Group 4 (p = 0.02). During HM, melatonin concentrations were significantly reduced in all groups compared to LM (Group 1: p < 0.01, Group 2: p = 0.02, Group 3: p < 0.01, Group 4: p < 0.02). Morning melatonin concentrations were significantly higher after HM in Groups 1 and 4 than after LM (p = 0.02, p < 0.01). Additionally, HM delayed melatonin onsets in Groups 1, 3 and 4 (p < 0.001, p = 0.02, p = 0.001). Group 3 preferred the light color during HM compared to LM (p < 0.01). There were no further significant light effects on visual comfort. Conclusions: We have first evidence for an isolated melanopsin -dependent impact of evening display light prolonging sleep latency and delaying melatonin onset. Further-more, relatively low melanopic irradiances of evening display light elicited NIF effects in the evening and morning. Therefore, since many people are exposed to display light in the evening and at night, reducing melanopic radiance may be a crucial key to reducing unwanted NIF effects of light. Using metameric display light allows manipulating melanopsin excitation independent of visual appearance.
