Bright light and good moods...

We are more agreeable when the light is brighter: here is the abstract, and one figure, from "Exposure to bright light is associated with positive social interaction and good mood over short time periods: A naturalistic study in mildly seasonal people" published in the Journal of Psychology:

Bright light is used to treat winter depression and might also have positive effects on mood in some healthy individuals. We examined possible links between bright light exposure and social interaction using naturalistic data. For 20 days in winter and/or summer, 48 mildly seasonal healthy individuals wore a light meter at the wrist and recorded in real-time their behaviours, mood, and perceptions of others during social interactions. Possible short-term effects of bright light were examined using the number of minutes, within any given morning, afternoon or evening, that people were exposed to light exceeding 1000 lux (average: 19.6 min). Social interactions were labelled as having occurred under conditions of no, low or high bright light exposure. Independent of season, day, time, and location, participants reported less quarrelsome behaviours, more agreeable behaviours and better mood when exposed to high but not low levels of bright light. Given that the effects were seen only when exposure levels were above average, a minimum level of bright light may be necessary for its positive effects to occur. Daily exposure levels were generally low in both winter and summer. Spending more time outdoors and improving indoor lighting may help optimize everyday social behaviour and mood across seasons in people with mild seasonality.

Figure - Quarrelsome behaviours (a) and agreeable behaviours (b) during time periods of no, low, or high levels of bright light exposure. Values are expressed as estimated least squares means of ipsatized frequencies, multiplied by a factor 100. Error bars represent standard errors of the mean. (* Significantly different from social interactions with low bright light exposure.)