Then half the winter mice stayed in a winter cycle, while half switched to a summer schedule. The summer mice were similarly split. The mice raised in equal periods of light and dark were split into three groups, one of which stayed on the 12-hour schedule, one of which joined the winter group, and one of which joined the summer subset.

　　After 28 days, it turns out the summer-born mice behaved the same whether they stayed on the summer cycle or switched to winter. But among the winter-born mice, those stayed in winter kept their previous schedule, while those that switched to summer stayed active for an extra hour and a half, which indicates that mice born and weaned in a winter light cycle showed dramatic disruptions in their biological clocks.

　　The finding is the first of its kind in mammals, and it could explain why people born in the winter are at higher risk for mental health disorders including bipolar depression, schizophrenia and seasonal affective disorder.

　　“We know that the biological clock regulates mood in humans,” said study researcher McMahon. “If an imprinting mechanism（运行机制）similar to the one that we found in mice operates in humans, then it could not only have an effect on a number of behavioral disorders, but also have a more general effect on personality.”

　　41. How many groups of mice are there finally in all in the experiment?

　　A. four

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