At least one man in Greenbelt sleeps naked, with only a sheet for cover. His wife sleeps in knee socks, a full-length flannel nightgown, and a quilt doubled over her for added warmth.

A Silver Spring couple drove to Boston over the holidays. She had her passenger's side heat vents fully open while he drove with his window down.

Today, with most buildings heated to only 68 degrees in the winter and cooled to 78 in the summer, one usually is too hot or too cold.

In one Capitol Hill office, the male staff director turns down the thermostat as soon as he enters the office, then exits for morning paper and coffee. A female lawyer sneaks over and turns the temperature up. Flicking of the dial continues until dark each winter day.

Kathy Beal of Chevy Chase says, "Now that I'm 42 and pregnant, the least I expect is a warm bedroom. But we spend $500 a month on heating oil so we set our heat at 55 degrees. My husband, Ted, sleeps in jogging suit and robe--real romantic! For him, the cost is more of a factor than comfort."

Gene Peters of Darnestown is the family member most often chilly. Wife Betty cajoles him into splitting wood for the fireplace to help raise his body temperature. Gene says, "Most men who claim they're hot aren't being honest. They're really chilly but put up with it because they pay the heating bills. In our house, Betty pays the bill so she's the one who turns it down to 55 degrees. I sneak it back up when I can."

The Peters have a pet parrot ("Polly") requiring tropical temperatures for survival. In the bird's room, which houses the thermostat, is a space heater. In the rest of the rambling, country home, Betty, children, and visitors wear sweaters and wool socks. They ignore Polly and Gene's adjacent sauna-like room.

"Whether you are a hot or cold person has a lot to do with your body's own rate of heat production and heat loss," explains Dr. Ralph Goldman of the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. "There are such great individual differences," he says, "that even in the so-called 'comfort' ranges, only 90 percent of the population is comfortable."

Body heat comes from food calories, and the faster your body burns the food, the warmer you feel. Eating a big meal, for example, temporarily speeds up the rate at which calories burn. Dieting often leaves you feeling extra chilly since your body uses limited food more slowly.

Movement also turns up the heat by making your body burn calories faster. "Even fidgeting may use up an extra 25 calories an hour--enough to warm you as much as raising the thermostat three degrees," says Dr. Goldman.

The reason most women shiver through football games while their male companions feel comfortable is that women have a higher proportion of fat than men. Fat, it turns out, doesn't keep a person warm, because it's inactive. Muscle, however, is active tissue. It burns up calories. Chances are the more muscular you are, the warmer you feel. While fat does insulate your inner parts, the layer of padding keeps heat away from skin and makes you feel colder.

At any threat of cold, blood vessels near the skin contract. This process, called vasoconstriction, keeps warming blood deep inside your body to protect vital organs, especially if you become really chilled. But vasoconstriction leaves the skin feeling chilly, and it's your skin temperature that tells you how hot or cold you feel.

In some cases, the temperature of the arterial blood flowing to the hands and feet may be reduced by as much as 15-25 degrees before the blood reaches the extremities. The hands and feet may even attain the same temperature as the surrounding environment.

The feet constitute approximately 10 percent of the total body surface. Under hot conditions, approximately 13 percent of the body's heat can be lost through the feet. But under cold conditions, when the heat loss might be expected to increase, it may actually be diminished and amount to only 7 percent of the total due to reduced blood flow and reduced blood temperature.

The body's heat loss can also be controlled by changing the amount of body surface exposed to the environment. Because extremities have more skin surfaces than other body parts, they cool fastest and give you the first hint that you're cold.

In attempting to protect ourselves from the cold, we might be tempted to concentrate on insulating the extremities because they are areas through which considerable heat loss occurs. But if the body's trunk is amply protected, heat loss in lightly protected hands and feet may be tolerable.

Because heat is conserved in the torso and head at the expense of the extremities, the rate of blood flow (as well as the blood pressure) often rises in the cold.

Glandular workings depend partly on your genes, but they are affected by emotions. Stress, for example, speeds up the adrenal glands, which causes you to feel cooler, because they regulate vasoconstriction. According to Dr. Jan Stolwijk, professor of epidemiology at Yale University School of Medicine in New Haven, the more active your thyroid, the warmer you may be because this gland controls how fast you burn food.

"Vasoconstriction will even take place sooner to protect you if you expect to be cold," says Dr. Goldman.

In addition to your attitude toward the cold, you can get used to it. The less acclimatized you are to heat and cold, the more extreme it will feel. "Ironically, if you're a Floridian, you could mind heat more than a New Englander because you're used to air conditioning," says Dr. Goldman. "And you'll find the first 20-degree day in December feels icier than the same weather in February because by late winter, your body has learned it won't freeze to death, and there's less vasoconstriction."

Dr. Frederick H. Rohles Jr., director for the Institute for Environmental Research at Kansas State University in Manhattan, Kan., believes that interior decorating can also affect how you perceive temperature. He reports that people thought a room was 2 1/2 degrees warmer when it had comforts like paneling and rugs.