Oct. 29, 2001 -- Sigh. The long, sunny days of daylight-saving time are over, and we humans are left to make the best of it -- more darkness, less light, for the next several months. Why is it we just want to sleep on those cold, dark winter mornings?
Turns out, that craving for sleep is part of our very human seasonal clock.
"Our bodies need more sleep during the winter months and would get more sleep if we didn't have fire and electricity" says Brian Prendergast, PhD, a postdoctoral fellow in psychology at Ohio State University in Columbus.
"One of the biggest human health issues is the invention of the light bulb," he tells WebMD. "Light is a very potent drug, not regulated by the FDA. We're already operating on a sleep debt, and when we switch on the light at night to stay up later, we're refinancing that sleep debt. I advise people to sleep more during winter. Our bodies want it."
For years, Prendergast has been studying the effects of seasonal changes on mice and men -- on the health and sexuality of both species.
His newest study looks at deer mice, shedding light on the effects of shortened days and the animal's immunity. The hormone melatonin -- only released during darkness -- seems to be at the heart of it all.
His study appears in the current issue of the journal Proceedings of the Royal Society of London: Biological Sciences.
For mice and other outdoor animals, environmental cues like shortened daylight hours indicate that winter's coming -- time to "thicken your fur, put on fat, hide food in the burrow," he tells WebMD.
It's all about sex and survival, he says.
"An animal can very accurately discriminate between a 12-hour and 14-hour day," he says. "Length of daylight is an animal's principal cue to increase or decrease the melatonin they secrete. Melatonin secretion becomes their internal calendar, the trigger to make adjustments for the next season."
"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.
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.