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A new study published in PLOS Genetics looked at how light affects behavioral despair involving the clock gene Period 1 using mice.

“The study is about how light may affect our brain,” study author, Dr. Urs Albrecht told us. “In particular, how light can influence our wellbeing and mood. We were wondering whether light really can affect the brain, because from personal experience life appears to be easier in summer when there is more light than in winter.”

In humans, there is a special form of winter depression, called Seasonal Affective Disorder (SAD), that can be treated with light. However, it is unknown how light does this. Researchers observed that their mice that have mutations in clock genes, displayed depressive behavior.

“We and others previously observed that some clock genes (e.g. Period genes) can be activated by light,” Dr. Albrecht told us. “Therefore, we hypothesized that these genes may be involved in the light mediated effect to improve depressive behavior.”

According to the American Psychiatric Association, the Diagnostic Manual of Mental Disorders (DSM-5) identifies SAD as a type of depression called Major Depressive Disorder with Seasonal Pattern. The most difficult months for people affected by SAD are the months of January and February and there are some people who experience SAD during the summer months as well. Treatment includes Cognitive Behavioral Therapy (CBT) and medication such as Selective serotonin reuptake inhibitors (SSRIs) which are antidressants.

The results told researchers that only Period1 (Per1) but not Period2 (Per2) was involved in the beneficial effects of light on depressive behavior.

“As a student, I always felt winter was much harder for me than summer,” Dr. Albrecht told us. “In particular, the place where I lived, Zürich, was very foggy during winter and there was not much sun. Hence, this correlation remained in my head until much later when I started to study the circadian clock and its ability to adapt to changing light cycles.”

To test their hypothesis, researchers used mice lacking the clock gene Per1 in all body cells or only in the lateral habenula, a brain region known to balance the reward system in the brain.

The results showed that mice with Per1 deleted in all body cells displayed depressive behavior that could not be treated with light. Removing Per1 in the lateral habenula only abolished the positive effects of light on depressive behavior compared to control animals.

“It was surprising to see the effect was so specific to Per1 and that lack of Per1 in the habenula only affected the light benefits on depression but did not make the animals depressive,” Dr. Albrecht told us. “This meant that depressive behavior in complete Per1 body knock out animals is probably related to other brain regions, such as the nucleus accumbens or ventral tegmental area, two other brain regions involved in the regulation of the reward system.”

These results indicate brain region specific functions of the Per1 gene (and probably of many other genes as well). Per1 in the lateral habenula is important for light effects on mood, Per1 in the suprachiasmatic nuclei is important for advancing clock phase. In the future, Dr. Albrecht says we will see which brain region is important for causing depressive behavior when the Per1 gene is lacking.

“Light perceived by our eyes does not only serve the purpose that we can see shapes and colors,” Dr. Albrecht told us. “It has hidden effects on our wellbeing and functioning of the brain. This is the reason why constant light with no sleep is probably effective as a torture method.”