Several recent studies have suggested that winter-born babies are more likely than summer ones to develop conditions such as schizophrenia, depression and seasonal affective disorder (SAD). One study may help explain why: the amount of daylight to which newborn mice are exposed sets the behavior of key biological clock genes for life.

A group of researchers from Vanderbilt University and the University of Alabama at Birmingham raised one group of mouse pups as if it were winter, giving them eight hours of sunlight a day, and a second group as if it were summer, with 16 hours of sunlight a day. Then they exposed them to either the same light schedule or the opposite for an additional four weeks. Compared with the “summer” pups, the “winter” pups’ biological clock genes were turned on for shorter periods regardless of the day lengths they were exposed to postweaning. The winter pups were also more active at night, similar to patients with SAD, suggesting their clocks were not as well aligned to the time of day. But don’t buy a UV light for the nursery just yet. Researchers are still working to determine what effect these seasonal signals have on humans.