Winter blues, spring fever—most of us take seasonal changes in mood for granted. According to a new study, the cause might be the seasons tinkering with the chemicals in our brain. As reported in the November 3 Journal of Neuroscience, researchers at the National Institute of Mental Health found evidence of seasonal differences in dopamine—a chemical messenger involved in motivation, pleasure, movement and learning.

Using brain scans, psychiatrist Daniel Eisenberg and his colleagues measured dopamine levels in the brains of 86 healthy people at different times of the year. People scanned in the fall and winter had an average dopamine signal 4.3 percent greater than those scanned in the spring and summer in an area that receives messages from dopamine-carrying neurons.

Eisenberg says future work will have to test whether dopamine levels in individuals fluctuate with the seasons the same way or if this result reflects some other difference unrelated to season between the groups, which were similar in age, sex and ethnicity. If the pattern holds, it means en­vironmental cues that change seasonally, such as the amount of sunlight we see, may actually mold our brain state.

Eisenberg says this type of dopamine fluctuation could contribute to the winter sluggishness and summertime pep experienced by many healthy people. It may also provide clues to the winter onset of seasonal affective disorder, as well as seasonal symptom changes noted in psychiatric illnesses such as schizophrenia.