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So far, we have introduced recommended points chronologically from the morning as guidelines for improving sleep habits, and this is the final rule of "The 10 Rules for Best Sleep."
From autumn onwards, when it gets colder, hands and feet tend to get cold. This is especially true for women who suffer from "cold sensitivity," and they are probably struggling. Some people with cold sensitivity who cannot sleep due to cold hands and feet wear socks to bed, but this is actually counterproductive. Wearing socks, contrary to their intention, makes it harder to fall asleep.
Therefore, this time, we will explain why wearing socks is counterproductive, as well as introduce countermeasures and ways to improve cold sensitivity.
Disadvantages of wearing socks
When your feet are cold and you can't sleep, one option is to wear socks. If you keep wearing them as you are, or if you go to bed with them on, heat will accumulate in your feet, preventing heat from dissipating from your hands and feet, which is important for lowering your core body temperature and facilitating sleep onset. This will make it difficult for you to feel sleepy and prepare for sleep.
If you wear socks, it is important to take them off before going to bed once you are warm.
What is cold sensitivity?
Humans are homeothermic animals, so their homeostasis works to keep their body temperature constant, usually around 37 degrees Celsius. This body temperature refers to the temperature of the central part of the body (core body temperature), where various organs essential for life activities are gathered. To prevent large fluctuations in core body temperature, the body adjusts blood flow, sweats, and shivers to maintain it.
For example, when it's hot, the body tries to lower its core body temperature by dilating peripheral blood vessels in the hands and feet, which have a large surface area, to increase blood flow and promote heat dissipation from the hands and feet. It also sweats to dissipate heat by utilizing evaporative cooling. Evaporation accounts for about 90% of heat dissipation in such cases (※1).
Conversely, when it's cold, to prevent the core body temperature of the important organs from dropping, the peripheral blood vessels constrict, reducing blood flow, and the body tries to retain blood in the core to maintain body temperature.
Coldness is a condition that occurs when this thermoregulatory function does not work properly. "Cold sensitivity" generally refers to "a chronic coldness in a part of the body that causes uncomfortable symptoms even at temperatures where one would not normally feel cold" (※2, 3).
The causes are thought to be low muscle mass and irregular female hormone secretion, as well as autonomic nerve dysfunction, poor blood circulation, and disturbed skin sensation, as more than half of women are said to have cold sensitivity.
Cold sensitivity is often associated with winter, but it can also occur in summer due to strong air conditioning or excessive consumption of cold food and drinks.
Coping with cold sensitivity
The phenomenon of not being able to sleep due to cold hands and feet when experiencing cold sensitivity is thought to occur because peripheral blood vessels in the hands and feet constrict when feeling cold, making it difficult for heat to dissipate. This prevents the core body temperature from dropping, which is necessary for sleep onset, thus making it difficult to feel sleepy.
For sleep onset, a drop in core body temperature (specifically, the skin temperature rises and core body temperature drops, with the two temperatures narrowing to within 2 degrees) is necessary. However, first, it is necessary to warm the hands and feet, which have a large surface area, to raise skin temperature (and core body temperature) to promote heat dissipation.
One option for this is to wear socks. Wearing socks warms your feet, but here's what you need to be careful about: you should not continue to wear them as they are. Since you are warming them to dissipate heat, you need to take off your socks once they are sufficiently warm. By doing so, heat dissipation will begin, your core body temperature will drop, and you will be ready for sleep. Socks are designed to prevent heat from escaping, so heat dissipation does not occur when you are wearing socks.
Notes
Methods such as using electric blankets, hot water bottles, or kotatsu can also be employed, but if they continue to warm, a phenomenon called "hyperthermia: a state where heat cannot escape to the outside and accumulates in the body" occurs, and heat dissipation no longer takes place.
Hyperthermia is often seen in children and the elderly who have poor thermoregulatory function, so caution is necessary. If hyperthermia occurs, change into light clothing, pour lukewarm water on the body, and expose it to the wind to allow heat to escape (※4).
Not just before sleep, but some people wear multiple layers of socks or tight underwear and clothing with strong elastic bands when they feel cold. However, this can impede blood flow and actually lead to coldness, so choose items with gentle constriction.
Effective steps to make it easier to sleep
As mentioned above, a drop in core body temperature and an increase in skin temperature are necessary for sleep. Therefore, if you are cold and cannot sleep, please try the following steps:
- Warm your hands and feet: Wear socks, take a foot bath, or massage/exercise your hands and feet.
- Facilitate heat dissipation: Take off your socks, get out of the foot bath, or stop massaging, etc.
- Avoid activities that excite your brain: Go to bed in a monotonous state (a relaxed and unstimulating state for the brain).
Hidden benefits of foot baths
I mentioned foot baths as a way to warm your hands and feet, and foot baths are definitely more effective and recommended than wearing socks.
In Rule 9 of "The 10 Rules for Best Sleep," we discussed bathing: taking a bath 90 minutes before bedtime if you have time (carbonated baths are even more effective), and showering if you don't have time. However, foot baths are more effective and work faster than socks or showers.
Heat dissipation is primarily driven by the hands and feet, which have a large surface area and well-developed capillaries. By promoting blood circulation in the feet with a foot bath to encourage heat dissipation, you can achieve similar effects to taking a full bath.
While bathing primarily utilizes the principle that core body temperature drops by the same amount it rises ("core body temperature elevation approach"), foot baths can be said to be primarily a "heat dissipation approach." Although the rise in core body temperature is less than with a bath, it does rise somewhat, and heat dissipation occurs effectively. And unlike bathing, it's fine to do right before bed.
Fundamental measures against cold sensitivity
One reason why more than half of women are said to suffer from cold sensitivity is low muscle mass and lack of exercise. Women tend to build less muscle than men, leading to less heat generation from muscle activity and reduced blood flow. In addition, if they work at a desk (in an air-conditioned room) during the day and don't walk much during their commute, they are more likely to develop cold sensitivity.
Furthermore, anemia after menstruation, low blood pressure, and decreased (disrupted) female hormone secretion due to menopause or stress can also lead to poor blood circulation.
Cold sensitivity is not limited to women; it is also seen in men and the elderly. A lack of exercise, unhealthy eating habits, irregular lifestyle, smoking, drinking, high blood pressure, or arteriosclerosis can all lead to poor blood circulation and thus "coldness." Obese individuals are also at a higher risk of sleep apnea syndrome in addition to coldness, leading to many factors that hinder good quality sleep.
The fundamental measure against cold sensitivity is, after all, to improve daily lifestyle habits. If you are not exercising regularly, try to do so. Incorporate the tips introduced in other articles to improve your sleep. If your symptoms of insomnia or other sleep problems are severe or if you don't see improvement, consult a sleep specialist immediately.
【References】
※1: Kango-roo!, One Point Physiology, Heat Dissipation | Body Temperature and its Regulation
※2: Terumo Body Temperature Research Institute, Why does "cold sensitivity" occur?
※3:Health and Medicine Information Bureau, Body Symptoms, Causes of Cold Hands and Feet (Cold Sensitivity)
※4: UMK TV Miyazaki, Stay healthy in summer... Beware of "hyperthermia"! (Broadcast on August 20, 2018)
