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The Big Sleep

The morning air is crisp with the approach of winter. You lie in bed, warm beneath thick blankets, and reach for the alarm clock. Wouldn't it be nice to drift back into a deep sleep until . . . the first warm days of spring?

Alas, that's not an option; we humans must face the challenges of winter. But there are some creatures that cope with the cold temperatures and the food shortages by taking very long naps. Whether or not we can call their behavior hibernation, though, depends on a number of factors.

When most people think of hibernation they picture bears. Yet bears aren't true hibernators; their long nap is more properly called winter lethargy. A true hibernator, like a chipmunk, can reduce its body temperature to nearly freezing during hibernation and change its heart rate from 350 beats per minute to as low as 4 beats per minute within hours of retiring to its den.

The heart rate of a bear also drops, though not as rapidly. During the early part of its winter dormancy, a bear's heart rate averages 50 beats per minute. After several months of uninterrupted sleep, the rate may drop to as low as 8 beats per minute. But a bear's body temperature remains nearly normal during this period. That's the reason a bear can wake relatively quickly -- a fact that's resulted in more than one hasty exit by from a bear den researchers. Pregnant females wake in mid-winter to give birth, then go back to sleep while their newborn cubs nurse. Still, most bears sleep all through the winter if left undisturbed.

Rodents that exercise true hibernation, by contrast, wake every few weeks to eat small amounts of stored food and pass wastes. These brief periods of activity are extremely costly: up to 90 percent of the stored energy reserves (mostly fats) allotted for the entire winter are consumed during these bouts of arousal. Thus the animals that truly hibernate don't actually sleep all winter, while "winter lethargic" species often do.

The difference between these two strategies -- true hibernation and winter lethargy -- is related to the animal's size. Bears are too large to dissipate the heat necessary to enter hibernation, whereas smaller mammals, with their high surface-to-volume ratio, can achieve this temperature drop quickly and evenly.

Possibly the largest rodent that truly hibernates is the Woodchuck (also known as the Groundhog), and it's a champion napper. In the Northeast, it has been known to enter its burrow while the weather is still warm in September and not emerge until late March. In other words, a Woodchuck can spend more than half of its life sleeping.

Sound like a good plan? Would you like to doze off after the end of the World Series and wake up just in time for opening day? The concept isn't too far fetched; researchers are experimenting with the compounds responsible for inducing hibernation, and they're finding that even species that don't hibernate will respond to treatment with these hormones.