I read this recently on PWS and thought you may be interested...
"Two hamster studies, one in the
March 7, 2002 Proceedings of the Royal Society of London, and another in the
March 19, 2002 Proceedings of the National Academy of Sciences indicate that hamsters' immune systems get a boost when the days get shorter and they prepare for winter. Merely by shortening their exposure to artificial lighting from 14 hours per day to 9 or 10 hours hamsters increased their numbers of immune cells, recovered from fevers faster, and had a greater immune response to psychological stress."