The response of the human brain to light harks back to Cro-Magnon times, when sunrise signalled the earliest opportunity to leave your cave to get your breakfast without being pounced on by a carnivorous beast lurking in the shadows. In modern times, most of us are slightly less adept at settling into the circadian rhythm of our prehistoric ancestors, waking at sunrise and going to bed at sunset, but we have nevertheless retained the same response to light. When light enters the eye, it is detected by the retina, where a tiny population of cells that express the light-sensitive pigment, melanopsin, project out and signal to areas of the subconscious brain that regulate circadian rhythms, sleep and brain function.

While it is already common knowledge that changes in light cycles that disrupt circadian rhythms and sleep patterns, such as working shifts or travelling around the globe, are detrimental to mood and learning, the effects of irregular exposure to light in the context of normal sleep cycles are largely unknown. Researchers at Johns Hopkins and Rider Universities therefore set out to determine if such abnormal light exposures could directly affect both emotional state and brain function when sufficient good quality sleep had been banked away.

To do this, they exposed two groups of mice to different light schedules: one group was in a ‘normal’ 24-hour cycle (12h light, 12h dark), while the other was placed under a modified 7-hour cycle (3.5h light, 3.5h dark): the 7h group was therefore in a lit environment when they would normally have been in the dark. Both groups of mice slept for comparable periods of time, and achieved the same level of sleep quality. Yet mice living under the 7h regime didn’t respond with their normal pleasure when fed sugary treats (a sign of depression), and didn’t tend to seek out their wheel for running fun (a sign of lethargy). Increased levels of corticosterone in the blood, an accepted marker of depression, were also observed.

When mice from either the 24h or 7h schedule were challenged to navigate their way around a water maze, the aim of which was to locate a submerged platform that offered a restful and relaxing alternative to swimming about, mice from the 7h schedule performed much worse than 24h mice: they were slower at finding the platform and had trouble remembering where it was. Interestingly, the administration of anti-depressants to mice under the 7h regime was able to reverse all these behavioural difficulties.

This research demonstrates that abnormal exposure to light, for example during the Winter months when light is harder to come by, has a direct effect on how you feel and how your brain works, and may lend support to the use of phototherapy as a way to ameliorate these issues.

LeGates TA, Altimus CM, Wang H, Lee HK, Yang S, Zhao H, Kirkwood A, Weber ET, & Hattar S (2012). Aberrant light directly impairs mood and learning through melanopsin-expressing neurons. Nature, 491 (7425), 594-8 PMID: 23151476