A study involving adult male white-footed mice may help us understand seasonal dysfunctions such as seasonal affective disorder. The study found that those mice kept in artificial light conditions mimicking winter (8 hours of light per day) had impaired spatial memory compared to mice kept in “summer” conditions (16 hours per day). They also had, on average, smaller brains, with a proportionally smaller hippocampus, as well as changes in dendritic spine density in that region. Other types of memory did not appear to be affected.
Mice brains shrink during winter, impairing spatial memory
 Short Photoperiods Impair Spatial Learning and Alter Hippocampal Dendritic Morphology in Adult Male White-Footed Mice (Peromyscus leucopus). J. Neurosci.. 25(18), 4521 - 4526.(2005).