Abstract: As an essential timing factor, ambient light plays a vital role in synchronizing internal biological rhythms with external lighting and dark environments. Meanwhile, ambient light is also a critical mood regulator; its non-image-forming (NIF) effects on mood are especially concerned by researchers. Previous studies have demonstrated that illuminance, correlated color temperature (CCT), and wavelength of ambient illuminance are key physical factors affecting mood. Moreover, unusual lighting patterns such as short lighting periods, artificial lighting during the night, and constant lighting/darkness have destructive effects on emotion and mood that may induce affective disorders such as depression and anxiety.
To date, the conclusion that short-time light affects emotion is not quite consistent. Studies have found that the NIF effects of short-term light on mood was not only affected by the intensity and spectrum of light, but also modulated by exposure duration, timing, individual characteristics, subjective preferences, and gene types. In a period of time, the longer the subjects received bright light exposure and the earlier they received morning light exposure, the lower their depression score. However, in the short-term lighting experiment of simulating office lighting, prolonged exposure of bright light was not conducive to individuals' subjective emotional experience; while the positive effect of CCT on emotion may depend on prolonged exposure. It is worth noting that receiving light exposure at different time in a day can advance or delay the circadian rhythms; thus, the timing could also regulate light's emotional function. In addition, women were found to prefer higher illuminance and lower CCT than men; while young subjects were more sensitive to polychromatic light with shorter wavelength than older subjects. Compared with individuals with PER3^4/4 genotype, individuals with PER3^5/5 genotype were more sensitive to light exposure and had a higher risk of depression; the 5-HTTLPR polymorphism was also proved to regulate the effect of light on the functional connectivity of the prefrontal cortex in healthy subjects. Lastly, the mechanisms by which light affects mood are shown from two aspects. On the one hand, the intrinsically photosensitive retinal ganglion cells may project light signals to the brain areas responsible for emotion regulation. On the other hand, ambient light may affect mood by altering circadian rhythms, regulating hormone secretion, neurotransmission, and sleep.
Future research can employ neuroscience technology to simultaneously examine the changes of subjective emotional experience and objective neural activity under multi-levels of illuminance and CCT; and combine multimodal data such as subjective rating, behavioral measurement, physiological response, and neural activity to track the effects of ambient light on mood. Besides, except for the NIF of light, ambient light may convey specific emotional meanings via the visual system, thus leading to various visual experiences transmitted by illuminance or wavelength, or lighting mode (direct or indirect lighting). Therefore, whether the visual perception of light, dominated by rods and cones, also potentially contributes to light's emotional function and how to separate it from the non-visual effects could be a promising direction in future research.

Key words: Ambient light, mood, physical factors, light patterns, underlying mechanism