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Abstract:
Objectives/Introduction: Exposure to evening light from displays elicits changes in sleep and circadian rhythms. These non-image-forming (NIF) effects of light are mediated in part by the photo pigment melanopsin. Melanopsin has a peak wavelength sensitivity of 480 nm at the retinal level, close to the typical 460 nm peak in LED-illuminated displays. Here, we modulated melanopic radiance while keeping cone responses constant (= same visual appearance) to better understand the role of melanopsin signaling on sleep latency, subjective sleep quality, melatonin concentration and melatonin onset.
Methods: Seventy-two healthy male participants completed a 2-week study protocol. Volunteers were assigned to one of four groups that differed in luminance levels (27–280 cd/m2). Illuminance while using the display ranged between 7 and 89 lx and were comparable to dim light (Group 1), smartphones (Group 2), tablets (Group 3) or computer screens (Group 4). Each volunteer was exposed to a low melanopic (LM) and high melanopic condition (HM), starting 4 h before habitual bedtime. The LM and HM differed in melanopic irradiance (ca. 3-fold change), but were matched in terms of cone excitation. Polysomnographically assessed sleep latency was manually scored. To evaluate subjective sleep quality, the volunteers completed the Leeds Sleep Evaluation Questionnaire immediately upon awakening from the scheduled 8-h sleep episode. Before, during and after light exposure, salivary melatonin levels were measured in half-hourly intervals throughout scheduled wakefulness.
Results: Sleep latency was significantly longer after HM than LM in Group 4 (p = 0.02) but not in Groups 1–3. During HM, melatonin concentrations were significantly reduced in all groups compared to LM (Group 1–4: p < 0.01, p = 0.02, p < 0.01, p < 0.02). HM delayed melatonin onsets in Groups 1, 3 and 4 (p < 0.001, p = 0.02, p = 0.001). Subjective sleep quality did not yield significant HM vs LM differences.
Conclusions: We have first evidence for a selective melanopsin-dependent impact of evening display light prolonging sleep latency and delaying melatonin onset. Furthermore, already low levels of melanopic illuminance elicited NIF effects in the evening. Thus, since many people are exposed to display light in the evening, the selective reduction of melanopsin excitation may potentially reduce unwanted NIF effects of light without compromising visual appearance.
