(RxWiki News) City lights, computers, TVs, cell phones, night lights — we live in a world with plenty of dim light interrupting the darkness at night. This may not be good for our mental health.
A recent study has found that a group of hamsters acted more depressed and had changes in their brain while constantly exposed to a dim light at night.
When the light went away, so did the depression.
"Reduce nighttime light exposure."
The study, led by Tracy Bedrosian, of the Department of Neuroscience at Ohio State University Wexner Medical Center, aimed to find out whether being exposed to dim light in darkness all the time might play a part in the development of depression.
The researchers tested this in a series of experiments with female hamsters. The hamsters were split into two groups, both kept in identical conditions and fed the same food and water.
Both groups were exposed to 16 hours of bright light each day and 8 hours of darkness. However, one group was exposed to to light at a power of 5 lux during the 8 hours of darkness each night. Lux is a unit of measurement of light.
For comparison, the full moon at night (without other light pollution) emits approximately 1 lux of light, and the lights in a family living room at night emit approximately 50 lux.
The hamsters in the room with the dim light remained there for four weeks. Then they spent four weeks with regular darkness at night like the control group.
While the hamsters were exposed to the ongoing light, the scientists did tests for depressive symptoms in the hamsters. One test was a swimming test and another was a test on their preference for sugar water — both used in other experiments to identify depression.
They found that the hamsters exposed to the dim light had more symptoms of depression than those in the normal dark/light room.
However, a week after the hamsters had left the dim-light-at-night room and returned to a normal cycle of light/dark, they no longer showed the depression symptoms.
The researchers also looked at the hamsters' brains. They were looking more specifically at "cytokines," which are protein molecules that can affect cell function.
Cytokines that produce inflammation have been linked to depression in past research.
The researcher found that the amount of one type of cytokine called tumor necrosis factor (TNF) did increase in the hamsters' brains while they were chronically exposed to the light.
TNF is part of a class of cytokines that acts as a toxin to cells and can cause them to die.
The amount of a different cytokine, interleukin-1b, did not appear to change in the hamsters' brains with or without the light.
Once the hamsters had no longer been exposed to the light, their amount of TNF went back down.
Then the researchers tested what would happen when they gave the hamsters an injection into the brain that is designed to stop the actions of TNF.
They found that the hamsters did develop the depressive behaviors, even while exposed to the chronic light, with this injection.
The researchers concluded that ongoing exposure to even a dim light at night might increase the amount of TNF in the brain, which then may be contributing to developing symptoms of depression.
While it cannot be certain that this same phenomenon occurs in humans, the researchers suggested further research into the area.
William Kohler, MD, the director of the Florida Sleep Institute in Spring Hill, Florida, said the article does have potential applicability to humans, though it's important to know the hamsters' day-night cycles.
Humans are diurnal, which means active in the day and sleeping at night — the opposite of nocturnal. It's important to consider, Dr. Kohler mentioned, whether the hamsters are nocturnal, diurnal or something called "propuscular," which means active at dawn and dusk.
"But the article does raise some significant potential explanation of the neurochemical changes that may be occurring in humans in the dim light situation which most of us experience in our 24-7 society that we live in," Dr. Kohler said.
"Articles like this are important to potentially show the underlying pathophysiology of changes that are occurring in humans, including depression as this particular article was discussing," he said.
Scientists have long suspected that environmental factors are contributing to rising rates of depression. The authors of this paper point out that depression rates have increased at the same time that the use of artificial light at night has increased in the past 50 years.
Past research has also revealed links between artificial light at night and an increased risk of breast cancer, obesity and mood disorders, according this paper.
"Recent environmental changes, such as LAN exposure, may warrant more attention as possible contributors to rising rates of mood disorders," they wrote.
Depression has already been linked to disruptions in people's circadian rhythms, the "inner clocks" that regulate body processes.
Light is one factor that can stimulate, interrupt or otherwise alter people's circadian rhythms.
"Overall, our findings suggest that chronic exposure to low levels of light at night may be one contributor to rising rates of major depressive disorder in recent decades," the authors wrote. "Given the growing prevalence of light at night, attention must be given to the physiological effects of this circadian disruptor."
The study was published July 24 in the journal Molecular Psychiatry. The research was funded by the National Science Foundation and by a Department of Defense National Defense Science and Engineering Graduate fellowship.
One piece of equipment used in the study was provided by the company Xencor. The authors declared no conflicts of interest.