As the days continue to get shorter and colder, it’s likely that you or someone you know has started to experience seasonal mood changes.
Over 10 million Americans have SAD, which is a type of depression that affects people 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?
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don’t necessarily feel sad, but instead, deal with an energy crisis that leaves them feeling tired all of the time, withdrawn from social activities, unable to sleep well, and unable to concentrate or focus.
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, […] 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.
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.
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 dim light conditions. 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.
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.”
