"Without that internal cue to prepare for winter, the animal would wake up one morning and suddenly find that his feeding ground is barren and it's 20 degrees below zero," says Prendergast.
In his current study, Prendergast looked at whether the animal's immune system also tunes into daylight changes.
He exposed deer mice to varying levels of artificial light. Some mice lived with long days -- 16 hours of light per day -- for 32 weeks. Another group of mice had short days -- eight hours of light per day -- for 32 weeks. A third group had long days for 20 weeks followed by short days for 12 weeks.
The mice were tested for immune function throughout the study.
The mice that were exposed to short days for only 12 weeks scored higher in immune function compared to the other mice. Other mice exposed to short days also had enhanced immunity. However, after 32 weeks the increase was gone -- because the internal clock -- the melatonin secretion -- signaled it was time to prepare for spring, says Prendergast.
Another interesting factor showed up: the male's testes (normally the size of a grape), shrink to the size of a half-grain of uncooked rice in the first six to eight weeks of shortened daylight. "This prevents them from breeding, and spares them the energy it takes to maintain nice, large testes, as well as the sperm and seminal fluid," he tells WebMD.
For mice, becoming "incapacitated" during winter translates into living longer. The male mouse is not out there expending energy during a time when food is scarce and he's living off his own body fat.
In essence, melatonin acts in animals like an "egg timer," triggering changes that will make sure the mouse's testes return to normal size in time for spring mating season.
"This enhanced immunity during winter months helps protect animals when they are most vulnerable to infection and death," he says. "They're living under stress, surviving on stored fat."
The message for humans: "Melatonin supplements may be doing you some short-term good, but in the long-term, they aren't working," he tells WebMD.