Holding a body close to you, it’s easy to appreciate the warmth a human body can generate.
Humans are “warm-blooded” animals. We’re able to effectively maintain a stable internal temperature, even on cold mornings or hot afternoons. This thermo-regulation is a dynamic process that balances heat generation (through metabolism and muscle activity) and heat loss to the environment, in order to maintain core temperatures.
In the first comprehensive experiments with a mercury thermometer in 1851, Carl Wunderlich demonstrated in studies of more than 25,000 individuals that the normal body temperature was around 37°C. He also noted that “grown up women may be a trifle warmer than men of an equal age”. Subsequent studies have suggested that if this difference exists at all, it’s imperceptibly small (less than 0.2°C).
However, there are a few situations where women’s core temperature are more clearly a little higher than men’s. Pregnancy and hormonal contraceptives will increase core temperatures by about 0.5°C to 1.0°C.
Core temperature also fluctuates between daytime and night time in a circadian rhythm: we’re generally 0.5°C to 1.0°C warmer in the late afternoon than in the early hours of the morning.
Temperature differences in bed
Interestingly, recent research suggests that men and women don’t use the same biological clock. On average, women go to bed earlier and wake up earlier than men. So these mornings can feel colder to women because it actually is colder at the time of their (earlier) waking.
And the night time nadir in core temperature, which occurs about three to four hours before waking, is over an hour earlier in women. So women have a head start: by the time their male partner eventually wakes up, women have been warming up for longer.
Our skin plays a particular role in regulating heat loss, chiefly by the control of perspiration (our own evaporative cooling) and blood flow to the skin. More blood flow makes the skin look redder, plumper and feel warmer to touch. But on cold mornings, these vessels constrict, reducing the volume of blood in the skin and heat loss to the environment.
While the core temperature remains within narrow parameters, the warmth of the skin is much more variable. On average, it sits around 33°C, but can vary a lot between different skin sites. The extremities (toes, fingers, lips, tip of the nose and ears) are the first to cool and can drop to below 25°C some nights if left outside the doona.
Unlike our core temperate, skin temperatures are clearly lower in women than in men, especially in cold air. One study, for instance, reported that the average temperature of women’s hands exposed to cold was nearly 3°C degrees lower than that observed in men.
This has been largely attributed to the very obvious dimorphism in body structure, limb proportions, surface area, insulating muscle and fat mass, thickness and distribution between men and women, which result in women maintaining a lower skin blood flow and, consequently, lower skin temperatures.
At the same time, our sense of how cold our bedroom is (or how warm our partner is) comes largely from skin temperature. By having cooler skin, women appear to be more sensitive to incremental skin cooling, meaning it takes less (cold) to reach a point at which skin sensors “feel the cold”, say enough is enough, and reach for the extra blanket.
Partly as a preemptive strategy, women (particularly younger, slimmer women) generally favour a higher skin temperature than men. Ultimately, this means that any gender differences in skin temperature under experimental conditions are eliminated in real life by prudent behaviour. If women feel chilly, they make themselves warmer. Men, by comparison, may be unfeeling but never cold.