In humans, 'winter blues' - the depression-like symptoms - occurs due to shorter days, but for rats it's just the opposite.
The depression-like symptoms known as "seasonal affective disorder," or SAD, occur when the shorter days of winter limit our exposure to natural light and make us more lethargic, irritable and anxious.
Biologists at University of California San Diego have found that rats experience more anxiety and depression when the days grow longer.
More importantly, they discovered that the rat's brain cells adopt a new chemical code when subjected to large changes in the day and night cycle, flipping a switch to allow an entirely different neurotransmitter to stimulate the same part of the brain.
Their surprising discovery, detailed in the journal Science, demonstrates that the adult mammalian brain is much more malleable than was once thought by neurobiologists.
Because rat brains are very similar to human brains, their finding also provides a greater insight into the behavioural changes in our brain linked to light reception.
And it opens the door for new ways to treat brain disorders such as Parkinson's, caused by the death of dopamine-generating cells in the brain.
The neuroscientists discovered that rats exposed for one week to 19 hours of darkness and five hours of light every day had more nerve cells making dopamine, which made them less stressed and anxious when measured using standardised behavioural tests.
Meanwhile, rats exposed for a week with the reverse - 19 hours of light and five hours of darkness - had more neurons synthesising the neurotransmitter somatostatin, making them more stressed and anxious.
"We're diurnal and rats are nocturnal," said Nicholas Spitzer, a professor of biology at UC San Diego and director of the Kavli Institute for Brain and Mind. "So for a rat, it's the longer days that produce stress, while for us it's the longer nights that create stress."
Because rats explore and search for food at night, while humans evolved as creatures who hunt and forage during the daylight hours, such differences in brain chemistry and behaviour make sense.
Evolutionary changes presumably favoured humans who were more active gatherers of food during the longer days of summer and saved their energy during the shorter days of winter.
