People born in winter are more likely to suffer mental health disorders, according to a recent study carried out by researchers at Vanderbilt University in Tennessee. Researchers raised baby mice from birth to weaning   in either "summer" light cycles of 16 hours of light and 8 hours of dark or "winter" cycles of 8 hours of light and 16 hours of dark. A third group experienced 12 hours of light and 12 hours of dark a day. Then half the winter mice stayed in a winter cycle, while half switched to a summer schedule. The summer mice were similarly _ . The mice raised in equal periods of light and dark were split into three groups, one of which stayed on the 12hour schedule, one of which joined the winter group, and one of which joined the summer group. After 28 days, it turns out the summerborn mice behaved the same whether they stayed on the summer cycle or switched to winter. But among the winterborn 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 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 the mechanism   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." How many groups of mice are there finally in all in the experiment?
Four. Five. Six. Seven.