Rats suffer their own version of seasonal gloom - brought on by brighter days, scientists have learned.
Just like humans laid low by the winter blues, the rodents are susceptible to a form of seasonal affective disorder, or SAD. But proving that the rat really is a creature of the night, it is excess light rather than darkness that disturbs the animals.
US scientists found rats experience more anxiety and stress as spring approaches and the days grow longer. Conversely, rats exposed to just a few hours of light a day experience a change in brain chemistry that lifts their spirits.
"We're diurnal and rats are nocturnal," explained study leader Professor Nicholas Spitzer, from the University of California at San Diego. "So for the rat, it's the longer days that produce stress, while for us it's the longer nights that create stress."
Rats explore and search for food at night, while humans evolved to hunt and forage in the daytime, said the researchers, whose findings are reported in the journal Science.
Over millions of years, this has led to hardwired differences in brain chemistry and behaviour. Natural selection favoured humans who were more active during the longer days of summer and conserved their energy in the winter. For rats, being most active under cover of darkness aided survival.
In a series of experiments, the researchers exposed rats to 19 hours of light and just five hours of darkness a day for a week. They found that the rodents' brain chemistry changed, increasing their stress and anxiety levels.
When the conditions were reversed, providing 19 hours of darkness and five of light, the rats perked up and their mood improved. The rodents became more willing to explore an open-ended maze and to swim - two tests that showed they were less worried and stressed.
Their responses to light and dark were due to a "switch" in the brain that toggled between producing two different kinds of nerve signalling chemical. One, dopamine, was boosted by darkness and served a "feel good" function, rewarding the rats with greater calmness and confidence. The chemical linked to light, somatostatin, did the opposite, making the animals fearful and edgy.
The scientists say they are still in the dark about how the neurotransmitter switch works, or precisely how it is triggered. But they believe taking the research further may help in the treatment of Parkinson's disease, which is caused by certain parts of the brain lacking dopamine.