Bright light affects the memory and learning centres of the brain, writes Barbara Bronson Gray.
When the sun goes down, you might end up much happier and better able to learn new things if you turn down all the lights — even your computer screen, say US scientists.
Their research was done just with mice. However, because the little rodents share the same set of special light-activated cells in their eyes — known as ipRGCs — that humans also have, it may be that the comparisons could apply to people, say the researchers.
Those cells, known as intrinsically photosensitive retinal ganglion cells, are stimulated by bright light, which affects the brain’s mood, memory and learning centres, they say.
Their study shows that chronic exposure to bright light at night elevates cortisol, a stress hormone that can cause depression and reduce thinking function.
The message: "Expose yourself to bright light in the day and avoid it at night," says study co-author Dr 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 is published online in the journal Nature.
Hattar says 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 says. 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 did not have the specialised ipRGC cells. Without them, the irregular light schedule did not impair mood and cognitive (thinking) function, 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 says.
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 behavioural changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment, according to the US National Institutes of Health.
One expert questions 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," says Dr Tony Tang, an adjunct professor in the department of psychology at Northwestern University in Evanston, Illinois.
He 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 says.
Scientists note that research with animals often fails to provide similar results in humans.
Study co-author Hattar says 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," he says. HealthDay News © 2012 New York Times Partner Publications
How the brain’s daily clock controls mood
A US mathematics professor is leading an international, $1m project examining the links between bipolar disorder and abnormalities in the circadian, or daily, rhythms of a mammal’s internal clock.
In humans, this grain-of-rice sized timepiece is a cluster of 20,000 neurons right behind the eyes. It is called the suprachiasmatic nucleus of the brain’s hypothalamus, and it is responsible for keeping your body in synch with the planet’s 24-hour day.
Scientists believe it is off kilter in patients with bipolar mood disorder (that was previously known as manic depression because of the typical, symptomatic violent mood swings). Some of the genes implicated in the disease are the same ones that regulate the biological clock.
The common treatment drug, lithium, is known to change the period of that clock. When bipolar patients are forced to stay on a 24-hour schedule, many experience a reprieve from a manic episode, says principal investigator Dr Daniel Forger, an associate professor in the University of Michigan’s department of mathematics.
Exactly how the brain’s clock controls mood remains a mystery, though. This new project aims to change that through complex mathematical modelling and experiments involving mice.
The researchers are continuously monitoring the state of the animals’ internal clock.
"We’ll watch it tick, use mathematics to understand its function, and test how it controls mood," Forger says.
The researchers will also be examining the brains of depressed and normal mice, and look for abnormal electrical activity. They aim to determine what state of the clock region corresponds with different moods in the animals.
"We’re going to learn an awful lot about the circadian clock, which could also, in addition to depression, play a role in Alzheimer’s, cancer and heart attacks," Forger says.