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I imagine that since the pandemic, more people have started taking their temperature regularly. It's generally recommended to take your temperature right after waking up for consistent readings, as body temperature rises after exercise or meals. However, some people might have experienced anxiety after waking up to a body temperature higher than their normal.
This article explains why your temperature might be high after waking up, the relationship between body temperature and sleep, and how to control your body temperature during sleep.
Is body temperature high upon waking?
Many might think, "Of course my temperature is high when I wake up," but actually, "waking up = high temperature" isn't always true. In fact, a person's body temperature is usually lowest in the morning and highest in the evening. While there are individual differences, if the temperature difference between morning and evening is within 1℃, it is generally considered that there are no health problems.
Body temperature fluctuates throughout the day
A person's body temperature does not remain constant throughout the day. Generally, it rises from morning until evening, reaches its peak, then declines, and is lowest in the early hours of the morning. This fluctuation is said to be about 1℃. As we age, temperature fluctuations become smaller, but maintaining appropriate changes in body temperature during the day and sleep is important for comfortable sleep.
Additionally, body temperature can change due to not only air temperature but also emotions, stress, and meals. Women's temperatures also vary with their menstrual cycle, so even if your temperature is higher than usual, there's no need to worry about your health if there are no clear changes in your physical condition. Since body temperature rises after meals, if you want more accurate readings, take your temperature once after waking up and once before dinner.
Does heat build up due to bedding and pajamas?
A high temperature upon waking might be due to heat accumulation in your bedding and pajamas, which subsequently raised your body temperature.
Heat buildup in bedding is caused by temperature and humidity. First, the temperature inside the futon changes with the seasons. In warmer seasons, not only is the outside air hotter, but people's bodies also become warmer compared to colder seasons, making it easier for heat to accumulate in the futon.
In winter, excessive cold protection measures can lead to heat buildup. While you might not feel hot immediately after getting into bed, if you pile on too many blankets, heat can gradually accumulate, making it difficult to sleep comfortably.
Furthermore, it is said that people sweat about one cup's worth of liquid overnight, so if the sweat-soaked futon is left as is, it becomes difficult for heat to escape efficiently.
If moisture doesn't escape from the futon, it can lead not only to heat accumulation but also to the growth of mold and dust mites, and cause restless sleep. Therefore, choose breathable bedding. And if possible, air it out in a well-ventilated area daily to reduce moisture.
What is a normal body temperature?
To manage your own health, it's important to take your temperature regularly and know your normal body temperature. The average normal body temperature for Japanese people is said to be between 36.6℃ and 37.2℃ (36.89±0.34℃), but this can vary individually and cannot be generalized. While some people consider 35℃ their normal temperature, others might have a normal temperature of 37℃. If your normal temperature is inherently low, you should suspect a fever even at around 37℃.
Also, body temperature changes with age. It tends to be slightly higher in children and stabilizes as they grow, but generally becomes lower in older age. When taking your temperature, consider your age when determining if it's within your normal range.
Furthermore, if you suffer from conditions such as hypertension or menopausal syndrome, you might experience hot flashes even without a fever. If you are concerned about hot flash symptoms, it is recommended to consult a medical institution.
At what temperature is it considered a fever?
A slightly higher body temperature than usual does not automatically mean a fever. Just as there are individual differences in "normal body temperature" at rest, the temperature at which a fever indicates poor health also varies. However, generally, 37.5℃ or higher is considered a medical "fever," and 38℃ or higher is considered a "high fever."
However, body temperature can rise due to various factors such as age, health conditions like illness, after exercise, or wearing heavy clothing. Therefore, observe your condition and retake your temperature after some time.
The relationship between body temperature and sleep
While "waking up = high temperature" isn't necessarily true, there is a deep connection between sleep and body temperature. There are essentially two types of body temperature. One is skin temperature, the temperature of the body's surface, which is the general temperature you measure in your armpit or mouth. The other is core body temperature, the temperature of internal organs including the brain, which is about 0.5℃ to 1℃ higher than skin temperature to sustain life.
When a person falls asleep, blood flow in the skin increases, raising the perceived skin temperature (the temperature of the body's surface) in the face and limbs. Heat is then released from the skin, causing the core body temperature (the temperature inside the body) to decrease. In other words, the core body temperature drops due to heat radiation from the hands and feet, and as the difference between core and skin temperature narrows, the body switches into sleep mode.
Furthermore, by lowering body temperature, the brain can shift into a relaxed state, making it crucial to lower core body temperature for good quality sleep. If your hands and feet are cold, blood vessels constrict, preventing proper heat dissipation, and the core body temperature won't drop, leading to difficulty falling asleep.
If you feel your sleep is shallow, try to maintain an ideal body temperature during sleep.
1: Room temperature adjustment
By paying attention to the temperature of your bedroom and the rooms you spend time in during the day, you can maintain an ideal body temperature.
It is especially important to pay attention to the bedroom temperature; ideally, it should be around 25-27℃ in summer, 15-18℃ in winter, and humidity should be 50-60% year-round. Additionally, the temperature inside the bedding is considered comfortable at around 30℃ and humidity at around 50%.
If the room or bedding is too warm from heating or bedding, it becomes difficult for the body to dissipate heat, preventing core body temperature from dropping. This can lead to difficulty falling asleep, shallow sleep, and a decline in sleep quality. To sleep soundly, adjust the bedroom temperature to be slightly lower and use breathable pajamas and bedding that doesn't trap heat.
2: Actions upon waking
Humans maintain a balance of sleep and wakefulness according to changes in body temperature. Core body temperature decreases during sleep, so to avoid disrupting this temperature rhythm, after waking up, move your body moderately to raise your core body temperature, which will help you wake up feeling refreshed.
If you linger in bed after waking, it can disrupt your body temperature rhythm and make it difficult to sleep at night, so be careful.
3: Expose yourself to sunlight
If your sleep-wake cycle is disturbed, open your curtains and expose yourself to sunlight for about 15 seconds as soon as you wake up.
Exposure to bright light immediately after waking not only resets your internal clock but also activates the sympathetic nervous system, which controls body temperature regulation. This is effective even on cloudy or rainy days.
4: Do not drink cold beverages upon waking
Drinking cold water immediately after waking can rapidly chill your internal organs, making it difficult for your body temperature to rise. It's important to rehydrate after sweating during sleep, so try drinking warm water, coffee, or tea heated slightly above body temperature.
5: Eat a proper breakfast
Skipping breakfast is not recommended as it can make it difficult for your body temperature to rise. To help raise your body temperature, eat a substantial breakfast to replenish calories. If you don't feel hungry in the morning, taking a walk or light stroll, which also exposes you to sunlight, can activate your sympathetic nervous system and might stimulate your appetite.
6: Take a proper bath
As core body temperature drops, sleepiness increases, so take a 15-minute bath in lukewarm water (around 38-40℃) about two hours before going to bed.
Improved blood circulation in the hands and feet makes it easier for the body to dissipate heat, lowering core body temperature and leading to better sleep. Additionally, the parasympathetic nervous system becomes dominant, increasing relaxation, which is also recommended for those whose sleep is shallow due to stress.
7: Use bedding appropriately
The temperature inside the bedding (futon) is ideally around 30℃, with humidity around 50%. If the bedroom temperature is too high because it's cold, and excessively insulating, moisture-retaining, or non-breathable bedding is used, the body temperature during sleep may become too high, potentially reducing sleep quality.
Using breathable bedding helps maintain an appropriate temperature inside the bed, supports proper body temperature regulation, and thus improves sleep quality. If you have trouble falling asleep, wake up in the middle of the night, or feel uncomfortable due to stuffiness in your bedding, consider reevaluating your bedding, such as duvets, mattresses, pillows, and pajamas.
