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Several studies demonstrated effects of light on affect via projections from the retina of the eye to the circadian clock or via projections to areas involved in mood and reward. Few field studies investigated how naturally fluctuating light levels affect positive and negative mood in everyday life, but none addressed two key components of the reward system: wanting and liking. To elucidate diurnal profiles and immediate effects of dynamically changing light intensity in everyday life, subjective wanting and liking were assessed using experience sampling, while continuously monitoring environmental illuminance. Using a smartphone and light sensors, healthy volunteers (n = 27, 14 females, 23.7 ± 3.8 [M ± SD] years of age) were probed for 1 week, 9 times a day, to rate positive and negative mood, and 6 novel dedicated questions each on subjective liking and wanting. The multiband light spectrum was continuously recorded from sensors worn on the chest and intensities were averaged over the intervals between subsequent probes. Mixed effect models were used to evaluate how time of day and light intensity modulated subjective ratings. A total of 1,102 valid observations indicated that liking and wanting peaked around 6 p.m. and increased, respectively, by 13 ± 4% and 11 ± 4% across an individual's range of experienced light intensities. More traditional mood questions were less sensitive to modulation by light intensity. Combined experience sampling and environmental monitoring opens up the possibility for field studies on light in disorders in which the reward system is highly relevant, like addiction, depression and insomnia. (PsycINFO Database Record

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