As the days continue to get shorter and colder, it is likely that you or someone you know has started to experience seasonal mood changes. Symptoms such as a loss of energy, a dip in mood, a lack of interest or trouble focusing can often be attributed to the lack of daylight we take in - and when they get too disruptive, can be indicative of a mood disorder called Seasonal Affective Disorder, or SAD.

More than 10 million Americans have SAD, which is a type of depression that impacts sufferers during the fall and winter months when access to light is limited. But how does light play such a significant role in moderating mental and cognitive health? Lily Yan, an associate professor of psychology and director of the Light, Emotion and Cognition lab, explains how light exposure impacts our mood, memory and motivation.

First of all, what is Seasonal Affective Disorder, and what are some associated symptoms?

SAD is a type of major depression that is characterized by a seasonal pattern of depressive symptoms for at least two years in a row. Though many of us may not meet the exact diagnostic criteria for major depression during the winter, we still may experience a lighter form of these symptoms, often referred to as the "winter blues." Most people who experience this type of depression don't necessarily feel sad, but instead, deal with an energy crisis that leaves them feeling tired all of the time, withdrawn from social activities, unable to sleep well and unable to concentrate or focus.

When did you start studying this topic, and how do you conduct your research?

My previous research focused on understanding our circadian rhythm (which is our bodies' natural 24-hour sleep-wake cycle) and how circadian rhythms are influenced by environmental lighting conditions. Since I started working at MSU in 2008, I began to explore how light affects emotions and cognition, as these functions are known to be impacted by the circadian system. In 2012, I received a grant from the National Institute of Mental Health to start the research program on Light, Emotion and Cognition.

The phenomena behind SAD has been known for decades, but I feel like there is still a gap in the literature around this topic: we don't know enough about how light affects our mental health. This research can be challenging, as it is difficult to use human subjects to study neurobiological mechanisms and most laboratory animals are nocturnal, which respond to light oppositely from humans. However, at MSU we have a very unique resource: a diurnal rodent model (meaning that they are awake during the day, just like humans)! Using this model, my research program aims to understand how light interacts with our brains at the molecular, cellular and circuit level.

How does light impact the mechanics of our brains underlying mood and cognition?

The dominant theory in this field is that light affects our circadian rhythm by training our brain's internal clock and keeping it synchronized with our environment. However, when our circadian rhythm is interrupted by changes in the light cycle, that can cause cognitive and emotional issues such as irregular sleep patterns and moodiness.

In addition to regulating circadian rhythms, prior research has demonstrated that seasonal lighting condition can influence the amount of neurotransmitters (like serotonin and dopamine) present in the brain - meaning that in summer months, your brain is actually storing more of the chemicals that make you feel happy, alert and motivated. 

When we transition to a dark, cloudy season from a bright, sunny season, there are changes happening in the brain at an anatomical  level. The results of one study, which included over 400 human subjects, demonstrate that the hippocampus - the part of our brains that controls many of our cognitive functions such as learning and remembering - is actually physically smaller during the winter and changes based on the seasons.

How does research with rodents inform larger questions about human emotion and cognitive functions?

When working with diurnal rodents, we find that a lot of their behavioral and neurobiological responses to light conditions are consistent with those of humans. When conducting this research, we only change one factor at a time, which is the amount of light or light intensity the rodents receive during the day. We've found that when limiting their daytime light exposure, it results in many behavioral changes. For example, the rodents struggle to feel pleasure and/or remember things. 

Rodents generally like sweet-tasting things, but after a few weeks in a winter-like dim light condition, they stop caring about eating things that are sweet and just go for whatever is most easily available. But in a regular condition with more light, they get excited about the sweet-tasting treats again and try to get them. Additionally, we see a lower sex drive in males housed in a dim light condition. The animals housed in dim light also have lower levels of serotonin and dopamine in their brain compared to those in bright light. These results help to establish diurnal rodents as a viable model to study the effects of light on the brain that are relevant to SAD in humans. 

We've also conducted research to test the impact of daytime light exposure on the rodents' spatial learning and memory. When navigating a maze, the rodents housed in a dim light condition struggle to remember the course, but the animals from a bright light condition are able to complete the maze. We also found that in dim lighting conditions, there are fewer dendritic spines (which allow neurons to receive information) connecting neurons together in the hippocampus. This may explain why it is harder to process and store information when we are exposed to less daylight. 

Further research has shown that a neuropeptide (a type of neurotransmitter) called orexin plays a role in regulating light-dependent changes in learning and memory. In a recent study, we gave rodents housed in a winter-like condition this neuropeptide every day in a row for five days, and  found that their ability to learn and retain new information improved significantly. On the flip side, when we gave rodents in a summer-like condition a treatment that blocks their ability to receive orexin, the rodents were impaired in their cognitive abilities. These results point to orexin as an important neurotransmitter in mediating the effects of light conditions. 

Our future work will aim to further elucidate the neural mechanisms underlying the impact of bright sunny days or dark gloomy days on the brain, from the level of gene expression  to neural circuitry. The diurnal rodent model offers an opportunity to answer those questions, which can be translated to understand SAD and winter blues in humans. 

Given this understanding of SAD and the winter blues, what are some ways to lessen SAD-like symptoms during the wintertime?

If you are looking to get a diagnosis or need help managing a case of SAD, my advice is to seek professional help from a mental health care provider first. However, if you're just looking to improve your energy or motivation level during the winter months, I definitely recommend spending more time outside. Even though the cold weather can make it difficult, outdoor lighting is still way brighter than indoor lighting, even on a cloudy or overcast day. You could also look into a light therapy box to make indoor lighting even brighter. In the future, I hope a better understanding of the mechanisms underlying the effects of light on mood and cognition will lead to the development of new strategies for treating SAD, as well as other types of depressive disorders and cognitive impairments. Until then, light will remain as one of the most effective treatments for SAD and winter blues. 

For my undergraduate classes, I always share the following quote from Albus Dumbledore: "Happiness can be found even in the darkest of times, as long as one remembers to turn on the light."