June 9, 2020
Written by: Claudia Lopez-Lloreda
We see sunlight as something that is beneficial for our health, both physical and mental. I know I get cranky whenever I don’t get enough sunlight. Whenever I feel overwhelmed or anxious, I go up to my rooftop to get some sunlight and it almost immediately makes me feel better. In fact, science backs up the idea that light can control mood and cognition. In the wintertime, people use light therapy, exposing themselves to artificial light, to treat depression associated with the long nights and lack of sunlight, a condition called seasonal affective depression (with the sad acronym SAD)1. So, does this mean all light is beneficial for human health? Not entirely. Whether light hurts or helps our health may be dependent on the timing of exposure.
For example, light exposure can be detrimental, particularly right as we are about to go to sleep. In this era of electronic devices, more people now use their devices, usually their phones, in the minutes or hours leading up to sleep. Studies have found that light-at-night, as the researchers technically call it, is associated with greater risk of depressive symptoms2, including in people who work night shifts3.
Though scientists saw that light-at-night is associated with risk of depression, they didn’t know whether it altered mood by directly affecting pathways of the brain that control mood or just by messing up the sleep schedule or other daily cycles known as circadian rhythms. Of course, if light messes up your sleep schedule, you might have mood changes, which could mainly be due to the lack of sleep, but not necessarily light exposure by itself.
Researchers from various institutions in China explored this question in a new study published in Nature Neuroscience4. They developed a specific experiment that allowed them to parse out the differences between exposing mice to light during the day versus during the night. They exposed mice to light for 2 hours during the night or during the day for three weeks.
Exposing mice to light during nighttime caused them to exhibit more depression-like symptoms such as not eating as much sugar, which indicates decreased pleasure seeking (anhedonia). They also stayed still for longer periods of time in a forced swimming test, a measure of despair. Meanwhile, exposing mice to light during the daytime did not change their behavior at all. Importantly, neither of the light exposure groups had altered sleep or circadian rhythms, meaning the effect on mood-relevant behaviors by nighttime light exposure was not caused by altered sleep or circadian rhythms, but rather by affecting some other process.
To determine how light was affecting mood, researchers first examined the cells of the eye. The retina of the eye holds important cells necessary for transforming random points in your visual field into a coherent image. These include rods, cones, bipolar cells, and retinal ganglion cells (Figure 1). Rods and cones are critical for image formation. But other neurons called intrinsically photosensitive retinal ganglion cells (ipRGCs) are important for non-image forming vision and behaviors, such as modulating sleep and awareness5.
When researchers got rid of these neurons, nighttime light exposure no longer caused depressive-like symptoms in the mice. But what was happening beyond the retina? First, the scientists examined where these ganglion cells were sending their connections using tracing experiments and saw that these cells are connected to a specific brain area called the dorsal perihabenular nucleus (dpHb).
A second experiment revealed the importance of the dorsal habenula in controlling the changes in mood. Using a powerful technique called optogenetics that allows researchers to selectively activate certain circuits, they activated the projections from the retina (specifically from the ipRGCs they saw were important in previous experiments) to the habenula. This elicited an anhedonic response in animals, meaning this circuit was important in controlling the development of depressive-like symptoms.
Additional tracing experiments painted a more detailed picture: the projections from the habenula then connected to and activated an important reward-associated brain area called the nucleus accumbens. So the researchers asked whether this specific circuit was activated during the exposure to light. By measuring the activity of the neurons, they saw that that these connections between the retinal ganglion cells, the dorsal habenula and the nucleus accumbens all became more strongly activated when mice were exposed to light at night than when exposed during the day.
When the scientists specifically inhibited these connections, they were able to prevent the depressive-like symptoms in response to nightime light exposure. Ultimately, targeting any step of this pathway blocked the behavioral changes induced by nighttime light exposure, meaning this circuit mediates this important process. These findings in mice could mean that this pathway also contributes to increased risk of depression in people exposed to light during the night. According to the Bureau of Labor Statistics, about 15 million Americans work a night shift6. Additionally, we are being increasingly inundated with light due to light pollution and phone use. However, an important caveat to translating the findings from this study to humans is that mice are nocturnal, so light-at-night exposure occurs when they are active. Nevertheless, when light appears during the ‘wrong’ circadian phase, it may cause depressive-like behaviors in both nocturnal and diurnal animals by acting through the same pathway. Therefore, this study provides a possible circuit to be explored as a way to understand our response to nighttime light exposure.
Cover image. From Flickr, public domain.
Figure 1. From CNX OpenStax on Wikimedia Commons, CC BY 4.0.
- Jane, O., Christopher, T., & John, G. (2003). Light therapy for seasonal affective disorder. Cochrane Database of Systematic Reviews. doi:10.1002/14651858.cd004516
- Bedrosian, T. A., & Nelson, R. J. (2013). Influence of the modern light environment on mood. Molecular Psychiatry, 18(7), 751-757. doi:10.1038/mp.2013.70
- Angerer, P., Schmook, R., Elfantel, I., & Li, J. (2017). Night Work and the Risk of Depression: A Systematic Review. Deutsches Aerzteblatt Online. doi:10.3238/arztebl.2017.0404
- An, K., Zhao, H., Miao, Y., Xu, Q., Li, Y., Ma, Y., . . . Xue, T. (2020). A circadian rhythm-gated subcortical pathway for nighttime-light-induced depressive-like behaviors in mice. Nature Neuroscience. doi:10.1038/s41593-020-0640-8
- Schmidt, T. M., Chen, S., & Hattar, S. (2011). Intrinsically photosensitive retinal ganglion cells: Many subtypes, diverse functions. Trends in Neurosciences, 34(11), 572-580. doi:10.1016/j.tins.2011.07.001
- Work Schedules: Shift Work and Long Hours. (2018, August 29). Retrieved June 06, 2020, from https://www.cdc.gov/niosh/topics/workschedules/default.html