How Light Leads to Darkness: A Neural Link Between Nighttime Light and Depression
Gone are the days when the setting sun meant your day was done. In the growing darkness, street lamps flicker on, light switches are flipped, and glowing screens illuminate our faces. Even when we try to unplug, urban light pollution seeps into the space around us.
It is known that exposure to light at night can negatively affect our mental health. Working night shifts, using electronic devices, and adjusting to daylight savings time can all lead to increased symptoms of depression . During summer months in the Arctic Circle when the sun stays out all night, suicide rates dramatically increase . But until recently, these effects were believed to come from disruptions in sleep and circadian rhythms. While these can be important contributing factors, what we didn’t realize was that light itself can also directly affect our mood.
Research on this idea actually began on a much brighter note. Light therapy has proven to be an effective treatment for mood disorders, including Seasonal Affective Disorder (SAD). But if daytime light can act as an antidepressant, why does light at night have the opposite effect, and how can our brains tell the difference?
Scientists in Dr. Tian Xue’s lab at the University of Science and Technology of China recently began to explore this phenomenon. The researchers first confirmed that exposing mice to two hours of light each night led the animals to show depressive behaviors, including signs of helplessness, reduced motivation, and inability to experience pleasure . It took three weeks to produce these effects, but symptoms also lasted for three weeks after the light exposure ended. Importantly, this experiment did not change the animals’ circadian rhythms, meaning the depression was not just a byproduct of a disrupted cycle, but actually directly triggered by the nighttime light.
To understand how light was affecting the animals’ mood, Dr. Xue’s group started their search at the eye itself. Lining the back of the eye is the retina, made up of many types of cells, each designed to detect a different kind of light. The group focused on a particular cell type called intrinsically photosensitive retinal ganglion cells (ipRGCs). The job of these cells is to measure how much general ambient light is in the environment, and send this information to the brain.
A few years earlier, scientists in Dr. Samer Hattar’s lab at Johns Hopkins University showed that one of the main brain areas these retina cells communicate with is a section of the thalamus called the perihabenular nucleus (pHb) . The pHb receives light signals from the retina and then sends this information to either the medial prefrontal cortex (mPFC) or the nucleus accumbens (NAc), two brain regions involved in regulating mood and depression.
These were important steps to understanding how light and mood can interact, but it was still unclear why nighttime light is so much worse for us. Dr. Xue’s group decided to follow up on this. They discovered that pHb neurons that send signals to the medial prefrontal cortex do so throughout the day or night, but pHb neurons that communicate with the nucleus accumbens send most of their signals at night. They realized the activity of this second group of pHb neurons is regulated by circadian rhythms, which tell the body what part of the day it is. The circadian system makes this group of cells less active during the day and more active at night. The result is that, unlike the medial prefrontal cortex, the nucleus accumbens receives information specifically about nighttime light. This suggests that the light-induced depression comes from the nucleus accumbens and its mood-altering effects.
The researchers then performed a series of experiments to confirm that this retina→perihabenular nucleus→nucleus accumbens pathway was really generating the depression. They found that nighttime light did not make the mice depressed if either part of this pathway (retina→pHb signals or pHb→nucleus accumbens signals) was blocked. But artificially stimulating either part of the pathway at night was enough to produce depressive behaviors, even when the light wasn’t actually on.
Why should this neural pathway exist? Dr. Xue’s group suggests that this neural circuit probably evolved to warn animals to avoid nighttime light, so they could protect themselves from predators and maintain healthy sleep cycles. But now that we live with so much nighttime light, these same neurons may be getting hijacked or overused in a way that significantly reduces our mood. Of course further work will have to be done to confirm that the same mechanism exists in humans.
These studies have made major progress in revealing how nighttime light can affect our mood. Understanding these neural mechanisms will allow us to consider new ways to prevent depression, as nighttime light exposure becomes harder to avoid in modern society. For example, we know these ambient light-detecting cells in the retina are most sensitive to blue light. So as you stare at the glowing screen in front of you, switching to a warmer-colored “night mode” may be a good first step.
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Cover image by Victoria Heath.