Why are women cold and men warm at night?

The reason is that the thermoregulation system for men and women is different: women are fundamentally better designed and are able to insulate themselves better than men.

Controlled studies have demonstrated that excess cold or heat, which can affect the entire population to a greater or lesser extent, are particularly important in cases concerning the elderly, whose ability to self-regulate their body temperature is less effective as they grow older. An increase in the bedroom temperature of up to a degree above the ideal temperature can cause 24% of sleep disorders in the elderly.

Dr. Acuña-Castroviejo and Dr. Escames

International Melatonin Institute (https://www.institutodelamelatonina.com)

What happens if we become cold whilst we sleep?

Sleep normally begins when the level of melatonin begins to rise after a certain time each day; this effect is called DLMO (Dim Light Melatonin Onset), and occurs around 20:00. When the melatonin reaches a certain value a decrease in body temperature is triggered (about 3 hours later). Both factors, an increase in melatonin and the corresponding decrease in temperature combine to initiate the sleep process and decrease brain activity. That is why at night our body temperature decreases and we can feel cold even in the summer, which causes us to cover ourselves even if only with a sheet, to reduce the loss of heat through our body.

That is to say, feeling cold whilst we sleep is normal and physiological, but easily correctable.

Temperature problems at bedtime - Neolife

What happens if we are too hot?

We can be too hot for a number of reasons, including over-layering when going to bed as we feel the cold as we noted above. This makes us uncover ourselves at night, often unconsciously, which can make us cold again.

Other causes of heat are pathological, such as a fever, or a result of hormonal changes, as is the case for menopausal changes, or caused by certain medications, apart from hyperhidrosis, a disease that causes excessive sweating. The increase in temperature can alter the quality of sleep if it is also accompanied by excessive sweating, that is conditional on the humidity of the room.

Ultimately, any changes in sensations from cold to hot can lead to a less deep sleep and a subjective (and objective) sensation in the morning immediately after a disjointed sleep.

How does our body react to very high or very cold temperatures at bedtime?

If the environment is very cold or we become cold at night, we tend to reduce our body surface area. There is also peripheral cutaneous vasoconstriction; which causes us to lose less heat from our body as the body tries to maintain a normal core temperature. Sheltering ourselves as much as possible we voluntarily attempt to reduce heat loss.

If it is very hot, the problem may be even greater; we stretch to increase surface area and there is greater cutaneous vasodilation to remove as much heat as possible through superficial circulation to the environment. If the ambient temperature of the room is very high however, the thermal gradient of the temperature transfer from the body to the environment is reduced which reduces the amount of heat lost. If, in addition, the ambient heat in the room is humid, the evaporation of water through sweating is greatly reduced to such an extent that the loss of heat through evaporation is also reduced.

In summary, the mechanisms for heat loss in conditions involving high ambient temperatures are not very effective, which can lead to further difficulties when we attempt to fall asleep. In increased temperatures sweating can also wake you up which then leads to damp sheets, which can in turn cause us to feel cold at night.

Can this affect our sleep? How?

Indeed, both cold and heat activate the physiological processes discussed above, which prevent the body from maintaining the correct temperature necessary to sleep without interruption. The cold often acts as a warning signal that increases brain activity to put in place voluntary mechanisms intended to contain the cold. Alongside these mechanisms the body has a number of involuntarily processes that are controlled from the thermoregulation centers of the body; in a state of excessive cold this commonly leads to shivering. An excess of heat can also place the body in a position to do the opposite. In addition to all this we must stress that excess heat can lead to sweating and the loss of fluid which can potentially produce a sensation of thirst, which will return us to an alert or wakeful state.

Controlled studies have demonstrated that these factors, which can affect the entire population to a greater or lesser extent, are particularly important in cases concerning the elderly, whose ability to self-regulate their body temperature is less effective as they grow older. An increase in the bedroom temperature of up to a degree above the ideal temperature can cause 24% of sleep disorders in the elderly. A decrease of one degree is also associated with a decrease in sleep efficiency. In both cases, in addition to the alteration in thermal perception, there is a decrease in the production of melatonin at night, which contributes to poor quality sleep.

Why is it that no couples can agree on the temperature at bedtime? If one is cold the other is hot?

This is because the thermoregulation system for men and women is different. Both sexes feel the ambient temperature differently and their bodies react differently to regulate their core body temperature.

Women maintain their thermal equilibrium by modifying the blood flow to the skin, which rapidly increases heat loss through thermal transfer from the skin to the environment. Therefore, when going from a cold environment (for example the street) to a hot environment (a cafeteria), a woman will take off her jacket, but after a while she may put the jacket back on because she feels cold again; this is because heat has quickly transferred to the environment through her skin and caused her body to lose heat rapidly, so she feels cold again. Men, on the other hand, have a lower peripheral thermoregulatory capacity and tend to remove or put on additional layers less often than women.

From here many different problems can arise, as the temperature at which air conditioning is regulated in public places has been calculated to reflect the ideal temperature for men (many years ago, when few women worked) and not for women, and this means that women tend to feel cold more often than men.

How can we reach an agreement?

Women have a greater capacity to reduce blood flow to the skin as well as greater subcutaneous fat tissue and so are better insulated than men. As a result another curiosity arises which is that women devote a greater proportion of their blood to maintain a constant temperature in their vital organs (heart, lungs, etc.), whilst men use their blood to keep their skin warmer at the expense of other organs and this also causes men to lose heat more easily.

For the reasons stated above, men shiver below 18 degrees and sweat above 31 degrees whilst women do not. Notwithstanding this women feel temperature changes more quickly than men.

That is to say, women are more sensitive than men because, in this sense, like in many others, they are better designed than men.

The solution, as always, is to create an ambient temperature during both winter and summer months that is a compromise between to hot and cold. The ideal temperature for the bedroom? A temperature that is not too cold or too hot: between 17 and 20 degrees. Any additional benefits associated to feeling cold or hot will be made in each case with the addition or removal of layers.

What clothes should we wear if our partner insists on sleeping like they are at the North Pole?

If the ambient temperature is very low, thick sleepwear (pajamas) that is breathable but warm, and a lightweight but insulating blanket (fleece) is an ideal solution. If we wrap up in thick but not very breathable clothing this will result in the retention of heat.

And what to wear if, on the contrary, we are too hot?

On the contrary, very soft, light breathable clothing or sleep naked only covered by a light sheet if necessary as this will prevent a sensation of nocturnal cold as your body temperature drops.

What is the ideal temperature for sleeping together? Because a bed for two is not the same as a single bedroom.

For a couple this is between 17-20 degrees. If you sleep alone, depending on whether you are male or female, you are free to select your ideal temperature.