As the cold, frigid air blows through Penn State this winter, women may be shivering a bit more than men.

Women generally don't reach the average 98.6-degree body temperature, said Nancy Williams, assistant professor of kinesiology. The average body temperature of women is actually in the low 97-degree range.

Larry Kenney, professor of physiology and kinesiology, said there are many reasons why women, on average, tend to feel colder at a given temperature than men.

A significant reason women don't have the same thermostat as men is because of the women's menstrual cycle, Kenney said. During the cycle a woman's body experiences changes in progesterone, which heightens the thermostat level. Women also experiences changes in estrogen levels, which lower their thermostat level.

"During the Follicular phase, the first 14 days of a woman's cycle, the body temperature is 1 degree lower than after the Luteal phase, the second half of the cycle," Kenney said.

The first phase indicates when a woman has progesterone from the ovaries and the second phase indicates the loss of it after ovulation, Williams said.

A human body's thermostat is at the lowest in the morning, around 2 to 3 a.m. It then reaches a higher level in the afternoon, and then it lowers again during the evening, Kenney said. This is because the cyclic rhythm of hormones to the center of the brain, known as the hypothalamus, determines a body's temperature.

Hormones affect body temperature because the changes occur with blood flow to the skin, which changes the amount of heat that dissipates into the air, Kenney said. When blood vessels are wider, it allows more space for warmth to flow to the limbs.

A person's body size is also a component of body temperature, Kenney said. A person with a smaller body size will have a slightly lower body temperature. A person who is heavier may be warmer in the cold weather because his body is more insulated. People that are fit tend to have a lower thermostat baseline, Kenney said.

Food also determines a person's thermostat level. After a person has eaten a meal, he will have a slightly elevated body temperature due to the specific dynamic action of food, Kenney said.