Mechanisms mediating the effects of light on sleep and alertness: current challenges

© 2020 Elsevier Ltd Light is an important environmental cue that exerts a direct and potent effect on vigilance states. In humans, light exposure increases subjective alertness and activates brain regions that are involved in promoting wakefulness. The spectral characteristics of these alerting effects of light are consistent with a role of melanopsin-expressing photosensitive retinal ganglion cells (pRGCs) in mediating non-visual responses to light. In photophobic nocturnal rodents, light exposure can be anxiogenic and increase alertness in some studies, but in other studies it clearly induces sleep and reduces body temperature, heart rate, and locomotor activity. In this review, we propose several factors that may influence whether light has an alerting or sleep-promoting effect in mice. These include preceding sleep history, preceding light history, as well as the behavioural context in which light stimuli are delivered.