The Brain Wave: The good, the bad and the ugly of modern light

IPads are really fun to use, with functionality ranging from watching Netflix to playing Angry Birds. Even beyond evening entertainment, light-emitting electronic devices are entering the realm of education through waves of e-books and applications that promote learning, and their utility explains their ubiquity in our lives. However, have we really fully considered the implication of such technologies beyond their use in work and play? This question has been raised many times before. Some say that technologies make us lazier and more dependent on electronics. Others say they take away from interpersonal relationships as people glue their eyes to the small screen of their mobile phones, fingers rapidly firing away text messages.

A few months ago Anne-Marie Chang and colleagues published a study on Proceedings of the National Academy of Sciences (PNAS) that showed evening exposure to artificial light from electronic devices can have adverse consequences on health. By having participants read from either an e-reader or a printed book for four hours before bedtime, the study found that using the electronic devices disrupted sleep, shifted circadian rhythms and decreased mental alertness the following morning.

Here is what we have to remember. The study measured the acute health effects only after five days of e-reader usage. For many people, electronic devices are an integral part of everyday life. In other words, we are dealing with chronic exposure to artificial light. If five days produced such adverse effects on our health, are there real health consequences from prolonged usage we just haven’t realized?

Light sensory cues are an important part of our lives, giving us the ability to consciously visualize the external world. Additionally, our circadian rhythms are coordinated in synchrony with the daily light-dark cycle. Thus, irregular patterns of light can result in severe health consequences by disrupting circadian rhythms and sleep, leading to depression, cognitive impairments, metabolic disturbances and increased risk of cardiovascular disorders. Beyond influencing circadian rhythms and sleep, aberrant light environments can directly cause depression and learning deficits, even in the context of normal sleep and intact circadian rhythmicity. Through both indirect and direct pathways, changes in light environment may contribute to the recent rise in depressive illnesses and obesity, considering the ubiquity of chronic exposure to destructive light conditions such as shift work, jet lag and artificial lighting.

How can light induce such severe clinical, psychological and social consequences? In mammals all light information is detected by the retina (in the back of our eyes) and relayed to the brain via retinal ganglion cells (RGCs) for higher-order processing. Although most RGCs receive light input from cells called rods and cones, a small subset of RGCs also have the intrinsic capacity to detect light in addition to rod- and cone-dependent pathways. Known as intrinsically photosensitive retinal ganglion cells (ipRGCs), this atypical class of retinal neurons project to the suprachiasmatic nucleus (SCN), a master regulator of circadian rhythms. The SCN then processes these light sensory cues to drive photoentrainment, allowing internal circadian rhythms to be synchronized with the solar day.

Interactions with the SCN and downstream targets may underlie the mechanism by which light influences health and physiology. For instance, the SCN may interact with areas outside of the nervous system to influence metabolism, which may explain why mouse models of jet lag show metabolic disturbances and increased risk of diabetes and obesity.

It is also unclear how aberrant light influences mental health to result in depression and learning impairments. Neuroanatomical tracing studies indicate that in addition to the SCN, ipRGCs also talk to the lateral habenula, a region known for its role in reward with strong implications in the pathogenesis of depression. Along with its previously explored roles in mood, the lateral habenula expresses specialized proteins involved in circadian rhythms, suggesting its involvement in the brain’s overall circadian system. Additionally, the molecular clock in the lateral habenula is temporally delayed compared to that of the SCN, implying a functional interaction between the SCN and the lateral habenula. Given its strong roles in depression, it is very possible that light may affect the SCN in such a way that influences circuits in the lateral habenula to drive depression behavior. Novel technologies that allow for mapping and functional probing of neuronal circuits, such as virus-mediated transsynaptic tracing and optogenetics (see the Oct. 9 Brain Wave column), could be useful for gaining insight into the neuronal mechanisms of light-driven emotional and behavioral output.

Meanwhile, what can we do about our computers and iPads? Certainly they are very useful tools, but we haven’t fully considered the implications for such technological development in the context of our health. The obvious solution is to reduce usage of light-emitting electronic devices, but due to work and social reasons, this is not always feasible. A possibility could be better design of monitors that emit longer wavelengths of light, as opposed to the blue-shifted light that we are used to, since blue-shifted light has the greatest influence on circadian rhythms. Better work and air travel policies could also decrease exposure to irregular lights.