Sensory input from the external world profoundly influences homeostasis and evokes complex behavioral output important for health maintenance and survival. One such critical sensory input is light, which allows us to visualize the outside world.
Beyond visual purposes, light also has a significant impact on the body’s internal clock, known as the circadian rhythm, which regulates sleep, appetite and other physiological functions that allow an organism to anticipate regular changes in the environment. This anticipation allows us to engage in behaviors that facilitate better adaptation to the outside world. For instance, our ability to synchronize with the daily solar cycle enables us to fall asleep at night and wake up in the morning, aligning specific behaviors with appropriate times of the day.
Due to the importance of light in our physiology, irregular light patterns have major clinical consequences on the health and well-being of individuals. This is especially true for those who travel across different time zones (resulting in jet lag) or those who work night shifts.
Several studies have reported that changes in light environment are associated with depression and cognitive impairments. Additionally, shortened day lengths during winter time cause a type of transient mood disorder known as seasonal affective disorder (SAD). Interestingly, light-based antidepressant strategies attenuate symptoms of depression that result from seasonal changes in light. Based on these clinical studies, it is clear that light exerts major functional significance on our mood and learning abilities.
One hypothesis that explains these observations proposes an indirect mechanism; abnormal light schedules disrupt sleep and circadian function, which are canonical mediators of mood and cognition. When we don’t get enough sleep, many of our cognitive abilities, such as learning and memory, are affected. Lack of sleep can also impact our overall mood, making us more cranky and stressed. Furthermore, studies of individuals who work night shifts indicate that aberrant light exposure disrupts sleep and circadian rhythms. Thus, it may be possible that abnormal light schedules cause mood and learning impairments indirectly by first distorting sleep and circadian function.
Interestingly, recent works from mouse models suggest that light directly regulates mood and cognitive functions independent of sleep and circadian disruption. Given the relationship between circadian rhythms and the external environment, there must be a mechanism that signals light input from the outside world to the internal clock for synchronization purposes, or photoentrainment.
For a long time it was believed that rods and cones were the only photoreceptors responsible for relaying light information from the eye to the brain. However, this assumption is not able to explain clinical findings that show blind individuals who lack rods and cones still retaining the ability to stay synchronized with the 24-hour solar day cycle. Independent of rods and cones, there must be other pathways that relay light input from the retina to the brain.
It turns out that there is a third type of photoreceptor cell called intrinsically photosensitive retinal ganglion cells (ipRGCs). While ipRGCs do not play a role in image formation, they are able to detect light and communicate this information to brain areas that control circadian rhythms (often called our “lizard brain”).
Strikingly, mice that lack ipRGCs are not able to photoentrain. Although the mice are still able to “see” due to the presence of rods and cones, they are not able to align their internal clock with the external light environment. In other words, our ability to form complex images of the outside world is not sufficient to detect light for synchronizing our internal schedule with the outside solar cycle.
New findings show that through ipRGCs, light is able to directly affect behavior without disturbing normal sleep and circadian rhythms. Using a chronic light exposure paradigm that neither disrupts sleep nor circadian function, the Hattar lab at Hopkins showed that excessive light during activity-rest cycles result in depression-like behavior and learning deficits in mice. Moreover, mice without ipRGCs do not develop light-induced mood and learning alterations, indicating that light directly modulates mood and cognition through ipRGCs. Unexpectedly, light by itself plays a major role in our well-being by directly influencing our mood and learning abilities.
As a whole, light has a profound impact on our quality of life. Through direct or indirect pathways that involve sleep and circadian rhythms, irregular light schedules can exert severe consequences on our health.
Yet many of us live a lifestyle in which light schedules are not healthful, from transmeridian travel and working night shifts to staying up late at night to surf the Internet on our laptops and tablets.
Given the recent rise in depressive illnesses and research findings that implicate irregular light patterns in mood and learning deficits, we must consider the role of light in mediating our health and physiology. Through a better understanding of the relationship between light and biology, we may be able to design better work and travel policies that attenuate the harmful effects of aberrant light, paving way for a healthier and more productive society.
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