The short days, the weak sun, and the warm beds of winter can wreak havoc on your fitness routine.

As the outside temperature plummets, so too can the will to brave the elements for outdoor exercise. Experts say as long as you layer up, drink up and tune into how cold is just too cold it shouldn't impact your fitness.

"Come with a different game plan for the winter," advises Jessica Matthews, an exercise physiologist with the American Council on Exercise (ACE).

"Motivation tends to wane a bit, so explore different activities, including things you can do in your own home. Have a flexible approach to your fitness routine as the weather changes."

If you're keen to exercise outside in all seasons, says Matthews, learn to dress for workout success.

"Wear layers. That's really important when it's cold outside and make sure you're not becoming wet. Heavy cotton soaks up sweat, so you might want to stick with wool or polyester: something water repellent."

Matthews suggests a first layer of lightweight synthetic.

"The second can be a little heavier, but still avoid heavy cotton," she said.

And don't leave home without hat and gloves. Heat loss from the head alone is about 50 percent at the freezing mark, according to ACE's safety tips for cold weather.

Keeping hands and feet warm is crucial because in cold the body shunts blood away from the extremities to warm internal organs at the center.

"In temperatures below zero, consider a scarf or face mask, kept loosely over mouth, to warm the air a little bit before it gets into your body," Matthews said.

Sometimes it's just too cold outside.

"Check the air temperature and wind chill factor before exercising in the cold," Matthews said. "Data from the National Safety Council suggest that when the wind chill factor falls below -20 degrees Fahrenheit (-29 Celsius), a danger zone exists."

Hypothermia is a potentially fatal condition. Warning signs include light-headedness, dizziness and lethargy.

"Those are the beginning stages," Matthews said.

Most people are not likely to forget to layer up in the cold, but they will more easily neglect to hydrate.

"It's too easy to forget to hydrate in the cold, and it's so important to stay hydrated in winter," said Matthews. "You are going to sweat."

She suggests drinking up to 20 ounces (0.59 liters) of water two or three hours before working out.

Deborah Plitt is a trainer with the Illinois-based equipment company Life Fitness, which recently broke down the calorie burn of 30 minutes of winter activities, such as sledding (250 calories), ice skating (230 calories), shoveling snow (230 calories).

"Of course there's a lot of wiggle room (in the calculations). It's based on an average woman of 145 pounds," said Plitt. "And sledding and snow shoveling depends on whether the snow is wet or fluffy."

Plitt said the point is to stay physically active, regardless of the weather, even if that means just taking the dog for a walk (125 calories).

"They're mixing things up; they're enjoying what they're doing," she said. "And believe it or not you burn more calories when you're shivering."