You may think you've perfected the art of the wintertime nap - especially after a big holiday meal - but the true professionals are already seriously sacked out.

Tucked beneath trees and dug deep into dens, bears, marmots and other animals are headlong into hibernation.

That means deep snoozing, a drop to near-freezing body temperatures for some animals, and a season of surviving on fat packed on during months of feverish eating.

The grizzly bear, perhaps the most celebrated hibernator, may take a breath once every 45 seconds, drop its heart rate to 8 beats per minute and survive for months without eating, drinking or excreting.

It's one of nature's most extraordinary tricks, but it's more than simply sweet dreams. It's keeping death at bay during the harshest months.

"By and large, it's an adaptation to a shortage of food," said Charles Preston, curator of the Draper Museum of Natural History in Cody, Wyo. "They've got to find some way to survive."

Elk and pronghorn simply wander to lower elevations in search of food. Wolves and coyotes eke out a living on what few scraps they can find near home. Some simply head for the Little Big Sleep. Squirrels curl up in burrows, waking up every week or so to snack.

Snakes slip underground for several months of lethargic stupor.

Insects hunker down inside trees or underground.

Even similar animals that share the same patch of land do things differently in winter.

Take the pika and the marmot. The pika, a rodent the size of a pint glass, stockpiles food in rocky slopes in summer and snacks on it throughout the winter.

The marmot, a larger rodent, consumes calories vigorously before falling into a deep sleep where its body temperature drops until it's almost frozen.

All of them, over countless generations and mutations, have found a way to adapt and squeak through frigid temperatures, piling snow and dwindling food supplies.

"There's more than one good way to solve an ecological problem," said Scott Creel, an ecology professor at Montana State University.

And grizzly bears are one of nature's best problem-solvers.

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There was a time when scientists debated whether bears were "true" hibernators, because they wake up often during the winter and their body temperature drops only by about 12 degrees.

Most other, smaller mammals enter a torpid state, where their bodies drop to just a few degrees above freezing and feel cold, limp and lifeless if handled.

"Bears aren't like that," said Chuck Schwartz, leader of the Interagency Grizzly Bear Study Team. "If they hear a noise, they wake up."

The prospect of waking a crabby grizzly in winter has kept more than a few scientists from learning more about bears' sleeping habits.

"There are reasons why we've never done hibernation studies," said Travis Wyman, a bear biologist in Yellowstone National Park.

Bears start preparing for hibernation long before temperatures drop and leaves change colors.

As summer wears on, an internal signal warns that winter is approaching and soon there won't be enough calories available to keep their burly bodies going.

They start an all-out feeding frenzy, feverishly packing on pounds in an obsessive state called hyperphagia.

By late September, pregnant female grizzlies start heading for dens, which are usually dug beneath tree roots or on hillsides that will eventually be covered in snow.

Males tumble into dens later. Nearly all are sacked out by the second week of December.

Some bears start to get into a sleep state even before they find their dens, Wyman said.

They wobble around on shaky legs with sleepy heads in a state some call "walking hibernation."

"It looks like they're drunk," Wyman said.

Once denned up, their bodies become sleeping machines, dutifully dialing back heart, lung and other functions that will allow them to survive for months.

Grizzlies cut their metabolic rate by about half and live off the fat built up during summer and fall. They also recycle urea, usually excreted through urine, and break it down into nitrogen, which is used to build proteins that help maintain muscle mass.

"It's amazing what their bodies are able to do," Wyman said.

Pregnant females have a few extra tricks.

Although they breed in the spring, a special adaptation delays fertilization of the egg until winter, when it's clear there's enough body fat for the mother and cubs to survive.

"If she's not healthy, she's not going to have those cubs," Wyman said.

Cubs are usually born in February, hungry, nearly hairless and about the size of a rat. They spend the next several months tucked against their sleeping mother and living off her milk.

They are typically the last to emerge from hibernation, sometimes staying inside until the last week of May.

Males are a little more restless. They've been known to shake off sleep, come out of their dens to feed on a winter-killed elk like a midnight snack and wander back to bed.

"We've had bears reported (out) every month of the year," Wyman said.

The dens themselves tend to be dug into north-facing slopes between 6,500 and 10,000 feet elevation.

Vegetation or snow helps keep the wind out and there's often a chamber just big enough for a bear to curl up and hold in radiated body heat.

"I'm 6 foot 2, 215 pounds and when I get in there I don't like dens. It's a little claustrophobic," Wyman said.