Lifetime changes in the biological effects of light on non-visual brain functions.

Biological effects of light go beyond vision and influence non-image forming (NIF) brain functions such as sleep, circadian regulation, mood and cognition. These effects are primarily driven by the third class of photoreceptors (called ipRGCs), which are maximally sensitive to blue wavelength light and confer a maximal effectiveness to shorter wavelength light on NIF functions. Despite the growing recognition of the importance of light's NIF effects, most research has focused on young, healthy adults, while the impact of light may change during the early development and aging of the brain. This narrative review describes how the NIF effects of light change across the lifespan from foetus development and childhood (when data is available) through adolescence, adulthood and into older age. We begin by discussing how age impacts ocular structure, particularly the changes in photoreceptors. Our main focus is on cognitive brain functions which we define as the brain activity sustaining cognition as well as performance as the output of this activity. We highlight that evidence remains fragmented and unevenly distributed, particularly over the pre-adult trajectory, and that quantification of the lifespan changes is mostly not available. We further consider pupil and melatonin responses to light and circadian entrainment to allow approaching quantitative differences between age groups. We conclude by proposing future directions to reach the quantitative estimates needed by researchers, clinicians, policymakers, and designers to reach light interventions geared to a specific age group.