Figure 1 The impact of photobiomodulation on brain function. (A) Schematic diagrams of the mechanisms behind how photobiomodulation (light; l = 600–1000 nm) may influence neuronal activity. When light hits the neurones “flush”, it activates various intrinsic cellular mechanisms. Light is absorbed by a chromophore (accumulated chlorophyll metabolites, interfacial nanowater and/or cytochrome c oxidase) that then prompts mitochondrial activity and hence ATP levels. This then leads to an activation of transcription factors associated with a change in functional activity (via functional genes). (B) Schematic diagrams of the potential significance of the effect of photobiomodulation (light) on brain activity. Transcranially, light can only reach, at best, the cerebral cortex and the large-scale networks. It has been suggested that light alters functional connectivity in the default mode network and other networks (e.g., central executive and salience). We suggest that the significance of this effect has evolutionary links, subserving a survival strategy; with exposure to light and outdoors, the brain can switch attention to any potentially dangerous situation more readily. The light does not necessarily switch the default mode network off (red) and the others on (green and bold text), but it would alter the efficacy of the process by influencing the functional connectivity between regions (thicker connecting lines). The light source has been shown as red (e.g., 670 nm) for the sake of clarity of presentation, but any light within the red to near infrared range should have a similar effect.