Longer Days Increase Depression, Anxiety in Rats

Summers are a wonderful time with warm, long days that lift the spirits. However, for rats, longer days lead to high levels of depression and anxiety.

The study on rats' behavior was carried out at UC San Diego, where researchers found that rats suffer from summer blues. This happens because rats' brains have to adjust to a new neurotransmitter when days get longer, and this chemical messenger makes the rats anxious and depressed.

"We're diurnal and rats are nocturnal. So for a rat, it's the longer days that produce stress, while for us it's the longer nights that create stress," said Nicholas Spitzer, a professor of biology at UC San Diego and director of the Kavli Institute for Brain and Mind, according to a news release.

Study results showed that rats exposed to longer nights (about 19 hours of darkness) and shorter days (5 hours of light) had high levels of the "pleasure hormone" or dopamine, which made them less stressed. However, rats that were given more sunlight had high levels of somatostatin, which made them more stressed and anxious.

Researchers also found that the difference in hormones wasn't due to the growth of new neurons, but because the existing neurons were secreting different neurotransmitters.

The study is published in the journal Science.

"Light is what wakes us up and if we feel depressed we go for a walk outside. When it's spring, I feel more motivation to do the things I like to do because the days are longer. But for the rat, it's just the opposite. Because rats are nocturnal, they're less stressed at night, which is good because that's when they can spend more time foraging or eating," said Davide Dulcis, first author of the study.

Anxiety and depression in rats was assessed using behavioral tests; anxious or depressed rats are less likely to perform tasks such as swimming or exploring a maze.