Since my retirement I often succumb to an afternoon nap on the nearest couch. I am perfectly comfortable when I drift off and the temperature in my house is thermostatically controlled, yet I frequently wake up with a bone-deep chill. In the interest of helping baby boomers enjoy their future retirement, what is going on?
• When you lie down to nap, your heat production will drop to a resting value. When you fall asleep, your metabolism will drop further to around 80 per cent of that resting value. The body tries to balance heat production with heat loss to maintain thermal equilibrium, so during your nap body temperature will fall and you will feel cold.
During the day, heat balance of the body will be upheld as a result of its normal thermoregulation. So when it gets cooler, your body will react by an involuntary increase in muscular activity, and you might even start shivering if the drop is too sudden. During sleep these normal thermoregulatory mechanisms react more sluggishly and that is the reason why you wake up cold.
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This has nothing to do with age, but is due to a combination of how tired you are and the room temperature. You could raise the room temperature to a level where your body would be at thermal equilibrium during sleep, but this would be uncomfortable when you later went about your normal activities – apart from the cost to your purse and the environment. It’s probably best to cover yourself with a blanket.
Hands and feet play a special role in normal moment-to-moment temperature regulation. When you are at thermal equilibrium with your surroundings, blood in the vessels of the hands will keep them warm. The warmest skin temperature in a warm person is in the fingers. This is due to small but effective heat-exchanging blood vessels in the extremities of warm-blooded animals, the . These supply the superficial veins of the forearms and legs with warm arterial blood, keeping the underlying muscles warm and effective, so the animal or person is instantly ready for fight or flight. They also serve as regulators of heat dissipation from the body.
“The warmest skin temperature in a person who is already warm is found in their fingers”
In sleep, this mechanism is also active, making a good and refreshing sleep difficult to obtain if your hands are cold. My mother taught me always to cover myself with a blanket if I was to take a nap, and to tug the hands of her grandchildren down under the cover if they were to fall asleep.
Leif Vanggaard, Retired surgeon-captain, Danish Arctic Institute, Hellerup, Denmark