Daylight-Saving Time: Time to Fall Back ... Into Bed?
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.
Because humans aren't "seasonal breeders," melatonin plays no role in our sexuality, he says. However, among populations that have limited food resources, fertilization may be successful only during times when food supplies are plentiful. "It's more an issue of energy, not of melatonin," he tells WebMD.
Winter actually seems to create other vulnerabilities in humans. Unlike animals, our immunity seems to get worse; we're more susceptible to viral and bacterial infections, says Prendergast.
For reasons researchers don't yet understand, more children with adult-onset schizophrenia are also born in the wintertime.
More people become depressed during winter, because there is less daylight. A condition called Seasonal Affective Disorder is marked by deep depression that begins in the fall and spontaneously gets better in spring. Prendergast says that follows the same pattern as what he has seen happen with the testes in his study mice.
Light therapy has proven to be one of the most effective treatments for seasonal depression. For some people, light therapy combined with an antidepressant drug is "quite effective," he says.