They did a systematic review of 15 studies in this topic. The studies included used light sources such as LED and laser producing light rays in the near-infra red (750–1400 nm) or red (620–750 nm) wavelengths. These rays could reach 2-3cm below the skull, which is enough to stimulate the dorso-lateral pre-frontal cortex. Centres such as this may be hypo functioning in depression. The Authors propose multiple physiological mechanisms. The primary mechanism might be direct photic stimulation of the mitochondrial cytochrome C oxidase, improving tissue metabolism by producing more ATP. They also stimulate the anti-oxidant production, reduce inflammation by decreasing the expression of IL-6, produce vasodilatation and increased oxygen supply and may also promote neurogenesis.
The article provides preliminary evidence from animal and human trials that indicate its anti-depressant efficacy and superior tolerability. The reported side effects are skin burns, fatigue, insomnia and headache. Unlike the bright light therapy, there is no risk of damage to the retina, which appears to be a big bonus, in addition it is versatile, it is used in medicine for the treatment of a variety of conditions such as muscle pain, wounds, neuropathic pain, and headache.
There is no doubt that the search for newer and safer treatments for depression is going to continue. If these newer modalities of treatment are associated with robust evidence, reduced cost, limited complexity and accessibility, their acceptance might be better among the mental health consumers. For now, it is still necessary to unequivocally determine its effectiveness, safety, and treatment protocols. The later may take decades as we saw in rTMS (Trans-cranial magnetic stimulation). More importantly, this area of photic neuro-stimulation is relatively unregulated which exposes consumers to a wide range of devices and treatment practices already available. Such usage might potentially cause more harm than good.
Reference: M.A. Caldieraro et al. Journal of Affective Disorders 243 (2019) 262–273