The Multifaceted Implications of Light for Psychological Well-Being and Mood
Light not only allows us to see but is also fundamental to our health and well-being. Several parameters of light exposure, such as wavelengths, intensity, and timing of exposure, all play an important role on its effects on psychophysiological functions. The way in which studies have previously quantified light (based on intensity), has been found to be inadequate since it does not consider the spectrum of light which influences non-visual effects. Among these non-visual effects, light has been found to have antidepressant effects, however, these effects remain inconsistent for non-seasonal depression and their underlying mechanisms remain elusive. The three research studies in this thesis investigated the effects of light on mood. The first study focused on the direct pathway between light and mood, aiming to predict mood outcomes based on the amount of intrinsically photosensitive retinal ganglion cells (ipRGC) stimulation from polychromatic light in healthy individuals. The second and third studies focused on the indirect pathways, exploring predictors and underlying mechanisms of mood improvements by means of sleep and circadian re-alignment in the context of non-seasonal depression. These studies explored the antidepressant mechanisms of monochromatic light therapy and predictive models of mood improvement. The results from study I (systematic review) suggest that ipRGCs may not be as involved in the mood improvement associated with polychromatic light. Drawing strong conclusions from these results are, however, cautioned. Mood metrics used across the studies were inconsistent and the light sources were not designed to maximally stimulate ipRGCs. The results from study II (open-label trial) support the notion that light therapy does have antidepressant effects in people with non-seasonal depression. The underlying mechanisms for these antidepressant effects may involve improvements in sleep initiation and daytime functioning. Individuals with difficulties with fallings asleep and waking-up may be those that respond most prominently to light therapy. The results from study III (randomized controlled trial) indicate that depression symptoms improved slightly more in the active light therapy condition as opposed to a placebo condition. Although this effect was modest across the overall group, there were considerable inter-individual variations in treatment response. The degree of improvement in mood was associated with improvement in pre-sleep thoughts and the circadian rhythmicity of skin temperature. Short REM latency and worst global subjective sleep were predictive of greater response to light therapy. Overall, further research is required to disentangle the involvement of the different photoreceptors in the mood response to polychromatic light in healthy individuals. Monochromatic light therapy for non-seasonal depression yields overall modest antidepressant effects. Clinical applications of light therapy may benefit from further research investigating differential effects in sub-groups of depression and underlying mechanisms in larger studies.
Light Therapy, Depression, Circadian Rhythms, Sleep, ipRGC, Mood
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Université d'Ottawa / University of Ottawa