Bright Light at Night Could Up Depression Risk, Mouse Study Suggests
WEDNESDAY Nov. 14, 2012 -- A new study suggests that when the sun goes down, you might end up happier and better able to learn new things if you turn down all the lights -- even your computer screen.
Unfortunately, the research was done just with mice. But because they share the same set of special light-activated cells in their eyes that humans have -- known as ipRGCs -- it may be that the comparisons could apply to people.
Those cells, called intrinsically photosensitive retinal ganglion cells, are stimulated by bright light, which affects the brain's mood, memory and learning centers, the researchers said.
The study found that chronic exposure to bright light at night elevates cortisol, a stress hormone that can cause depression and reduce thinking function.
"Expose yourself to bright light in the day and avoid it at night," suggested study co-author Samer Hattar, an associate professor of biology and neuroscience at Johns Hopkins University, in Baltimore. "That will keep the ipRGCs that affect mood from being activated."
The research was published online in the Nov. 14 in the journal Nature.
Hattar said the research team was initially interested in whether seasonal affective disorder (SAD) -- a form of depression people sometimes experience in the lower-light winter months -- applied to mice. They exposed mice to an alternating cycle of 3.5 hours of light and then 3.5 hours of darkness. The mice got depressed.
How do you know that a mouse is sad? They take less interest in sugar and move less in the cage, and they have trouble learning and remembering, Hattar explained. When the mice were given Prozac (fluoxetine), a commonly prescribed antidepressant, their symptoms went away.
To understand the role of the retina's neurological circuits in affecting mood, memory and learning, the researchers studied animals that didn't have the specialized ipRGC cells.
Without them, the irregular light schedule did not impair mood and cognitive (thinking) function, even though their vision and general light detection ability remained intact. This showed that light affects learning and mood directly through these special photosensitive retinal cells, Hattar said.
The researchers created light-exposure patterns for the mice that allowed the scientists to rule out the possibility that circadian rhythm and sleep disruption were responsible for the changes in mood and learning ability they observed.
Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism's environment, according to the U.S. National Institutes of Health.
One expert questioned whether the mice's normal circadian rhythm was indeed maintained. "Perhaps even though the overall sleep timing pattern remained intact, the quality of their sleep deteriorated," suggested Tony Tang, an adjunct professor in the department of psychology at Northwestern University, in Evanston, Ill.
Tang also found an important difference between how humans are exposed to light at night in modern life and how the reaction of mice to light was tested during the research.
"In the current study, the poor mice ended up having bright lights shining on them while they slept; but for humans in the past century, we've stayed up while we kept lights on, and then turned the lights off when we sleep," he said.
Scientists note that research with animals often fails to provide similar results in humans.
Study co-author Hattar said the study should be replicated in human subjects. "But even if it comes out not as clear as it did in mice, I think there will be some benefit for people to turn down their lights at night. I don't think there is any harm in it."
Learn more about sleep-wake cycles from the U.S. National Institutes of Health.
Posted: November 2012