13 mins read

The Ultimate Guide To Body Temperature

Body temperature is a vital, biomarker that can reveal deep insights into your current health.

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In this article:

Article Highlights:

  • Why track body temperature?
  • Sports performance
  • Body Temperature Monitoring Technology

What is Body Temperature?

The current temperature of your body indicates how well you are generating and radiating heat. Body temperature regulation is a critical function controlled by the hypothalamus (brain region in charge of homeostasis). Just a few degrees too hot, or cold, and critical biological functions stop working effectively – this is why temperature makes such a big difference to your energy and performance. 

Core body temperature is the temperature of your internal organs (heart, kidneys and brain) and is kept under tight control between approximately 36.5 °C – 37.4 °C.  It usually requires invasive procedures to measure accurately (which people generally don’t appreciate) so both doctors and devices usually measure surface temperature. Internal thermometers (in the ear or mouth) are able to take temperatures close to core body temperature, because they go slightly inside the body.

Surface body temperature, or peripheral body temperature, is measured on the skin’s surface and has a wider temperature range of approximately 31 °C – 37 °C.  This is commonly measured with digital thermometers, using heat sensors, in more accessible places like the armpit or forehead. But since the skin is our main mechanism for cooling and cold temperatures change blood flow, these measurements need to be calibrated to core body temperature and can therefore be inaccurate.

Your body temperature naturally changes throughout the day; the lowest recorded temperature (at rest) for the day is called basal body temperature and can be used to understand and predict hormonal changes. Age, activity and health status also impact temperature, which makes it a useful tool for understanding and optimizing your body.

A core body temperature between 36.5 °C – 37.4 °C indicates a healthy body, whereas a perpetually low body temperature signals slowing metabolism. Conversely elevations in body temperature can mean the body is reaching fatigue, known as cardiovascular drift in the running world, or an infection is being dealt with by the immune system. 

Why Track Body Temperature?

If you want to lose weight or increase your health and fitness level, monitoring your body temperature can reveal unique insights about what’s going on internally that you simply can’t see otherwise.

By tracking temperature you’ll discover trends in your health data that can provide invaluable information about your:

  • Metabolism – are you producing energy efficiently?
  • Weight – are you burning or building fat?
  • Performance – is your temperature impacting stamina?
  • Hormones – where are you in your cycle? (for the ladies)
  • Health – are you coming down with something?

Metabolic Rate

Metabolism involves a series of chemical reactions that create the energy your body needs. Metabolic reactions occur in tiny organelles within your cells, called mitochondria. They ‘burn’ food (macro nutrients like fat and carbs) using oxygen to create electro-chemical energy and heat. Some mitochondria (in Brown Adipose Tissue, BAT or Brown fat) are ‘uncoupled’ from producing energy and are ready to warm you up when it gets cold.

There is a close relationship between metabolic rate and body temperature; a person with a high metabolic rate will have a higher body temperature compared to someone with a lower metabolic rate. As body temperature goes up, so does metabolism. For each degree C in temperature we need at least 10% more oxygen to match the higher metabolic rate.

Spotting changes in your metabolic rate can help you prevent problems before they occur.

Weight Gain or Weight Loss

Body temperature is inversely proportional to body mass. As a person loses weight their metabolism slows down and their core body temperature decreases. Learning the differential between your highest and lowest body temperature gives you a snapshot into your metabolic health, and ability to lose weight, when monitored over time offers you’ll know if it’s improving or not. 

The reduction in body temperature, and basal metabolic rate, experienced when losing significant weight, can be felt in cold extremities. It can also indicate immune stress as weight loss mobilizes toxins usually stored in fat cells. If you’re trying to lose weight and your body temperature drops you will plateau. You can increase both metabolic rate, fat oxidation and energy levels with exercise and a few more calories to get you back on track.

Sports Performance

Measuring core body temperature is an essential tool for athletes wanting to reach the next peak in their performance. Movement produces heat that increases core temperature, so we sweat to cool down. But, the body uses up a tremendous amount of energy to keep the core temperature in homeostasis – energy that could otherwise be used for physical movement and running further or faster.  If we can’t cool quickly enough (due to dehydration or an extreme environment) this heat stress causes fatigue both mentally and physically.

Athletes can gain competitive advantage by training in challenging external temperatures and learning how to maximize thermoregulation. This should be done while continuously monitoring body temperature, to safely acclimatize to the external conditions. You can also use temperature monitoring to quantify when you’ve recovered from training (when your temperature returns to its baseline.)

Hormonal Cycles

The basal temperature method is the most reliable method for helping women predict when they will ovulate. Just before ovulation, the female body produces more progesterone which raises the body’s temperature; the closer to ovulation a woman is, the higher her body temperature, starting at 35.5 °C – 36.6 °C at one end of the cycle and rising to 36.1 °C – 37.2 °C during ovulation. 

These hormonal temperature fluctuations can impact how women respond to heat (exercise or externally applied). Women tend to have more fat and less muscle, impacting skin surface area and energy production, they also sweat less, but more efficiently. Hormone-related temperature changes can let women know when they’re most ‘ripe’ for babymaking, help identify underlying hormonal imbalances and support female athletes to enhance performance.

Health Status

Abnormal body temperature is a natural indicator of illness, but our response to the external environment, is personal; some people cope better than others. While there are guidelines to ideal (‘normal’ for humans) temperatures you need your own dataset to get the biggest benefits. 

When you continuously monitor your body temperature any fluctuation from your own ‘norm’ becomes obvious. A change in temperature can indicate oncoming illness, allowing you to deal with it faster (nourish up, take a break, visit the sauna).   Look out for fluctuations of 0.5 °C or more, increasing or decreasing in temperature; this is a sign that your body is under strain, especially if coupled with other symptoms.

About 75% of people will get hotter when they get sick, but the rest seem to cool down, this is why you can feel feverish but have no ‘fever’. It’s the fluctuation from your own norm that is more important than the direction of the change. Stress and strong emotions can also cause a change in your body temperature; extreme stress can elevate your temperature to a psychogenic fever. Monitoring body temperature will also help you know when you’ve recovered from illness, which is nice. 

Thermography (heat mapping the body) is slowly replacing mammograms for early breast cancer detection. Infrared thermography can map body temperature and ‘see’ restrictions in blood flow, changes in blood temperature and localized metabolic issues that are associated with disease (cancer, diabetes, neuropathy, vascular disorders). Keeping track of your temperature could reveal problems that can be prevented way before they need to be treated.

Optimum Body Temperatures

Just a slight change in your body temperature can yield significant results. Our bodies have different optimal temperatures depending on the activity. You can also purposefully elevate or reduce your temperature to access some ‘advanced features’ of your immune system.

Too Hot

When this happens, your skin’s blood vessels expand to transfer the heat away from your core towards the surface. Your body starts sweating to keep cool. 

  • 37.5 °C and above is considered an elevated body temperature.
  • 38.5 °C and above indicates the body has created a fever.
  • 41 °C and above causes fatal damage to the internal organs.

Hyperthermia is when the body produces more heat than it can get rid of (radiate), causing it to overheat. This happens at 38 °C and higher. This is a medical emergency, typically caused by heat stroke or more rarely as an adverse reaction to medications and narcotics.

Heat Stroke happens when the body cannot control its temperature by cooling down through perspiration so it continues to rise. It can rise as high as 41.1 °C in just 10 minutes, creating an emergency that can result in death, organ failure or permanent disability if not treated quickly enough. 

Fever happens when the hypothalamus in the brain recognizes intrusive microbes in the body and typically raises the body’s temperature over 38.5 °C to remove them more quickly. Fever is not a disease process, but a normal response in a healthy body “fever is positive evidence of an active immune system, revved up and helping an array of immunological processes working more effectively.”

Fever Temperature Ranges

  • 37.8 °C – 39 °C is considered a low-grade fever that is beneficial for the body.
  • 39 °C – 40 °C is considered a moderate-grade fever supporting the immune system.
  • 40 °C indicates the body is fighting off a more serious problem and will feel physically uncomfortable fever, at this temperature you may want to check in with a trusted healthcare provider (or see if your temperature monitoring device has got too close to something hot!).
  • 42 °C and above is a serious fever that can be dangerous and you should seek medical assistance immediately.

Too Cold

When your body gets too cold, your hypothalamus sends nerve impulses to the muscles that cause them to shiver and in turn - warm up. Your Brown fat also gets activated to give off extra heat.

  • 35 °C is considered a body temperature that’s too low; this is called hypothermia.
  • 27 °C and below can be fatal.

Hypothermia happens after a long exposure to cold temperature and the body cannot produce heat quick enough, resulting in dangerous body temperatures. Hypothermia can occur as high as 4.4 °C if a person has been sufficiently chilled by cold rain, sweat, or a cold water source.

Hypothermia eats through the body’s stored energy which further lowers body temperature, and can restrict a person’s movements and brain function. Not cool. It is a medical emergency that should be treated immediately. 


According to exercise physiologist Gerald Endress, 21.1 °C is the optimum temperature for exercising. Roughly between 20 °C and 22.2 °C is the goldilocks zone. Body temperature peaks in the afternoon for most people, so this is the ideal time to get deliberately sweaty as your body is more supple and has a bit more energy. 

Performing exercise when hot can deliver additional benefits if you keep the body within specific, yet highly personal temperature zones. After training in the heat performance is typically enhanced and with a good set of temperature data an athlete can spot when they tip into temperature zones that drive fatigue and even exhaustion. Athletes can perform something like a bleep test, but using heat, called a heat ramp test, to determine your unique optimal performance.

Zone 1 – Reduced Performance: too cold to perform, rare for sports, unless in cold water or icy terrain.

Zone 2 – Normal: this is the body’s usual temperature with circadian variations throughout the day.

Zone 3 – Peak Performance: temperature for maximum power output and sustained energy.

Zone 4 – Heat Fatigue: power declines as energy is wasted in cooling, pacing and active cooling needed. 

Zone 5 – Heat-Stress: far too hot, dangerous to be here! Risk of internal damage, stroke, collapse or death.

For strength training zone 2 is ideal, for cardiac or endurance athletes should remain in zone 3, and can edge towards the top of the zone monitoring for heat fatigue. With specific heat training (dipping into zone 4 and getting back into zone 3) you can improve heat tolerance (changing your personal thresholds) and get a performance edge over unconditioned athletes. 

As your body temperature increases so does your heart rate and this increase in effort (heart pumping faster) is not matched with more power, so the perceived effort is much greater. This is known as cardiovascular drift, and by training to increase ‘thermal load’ (the amount of heat your body can cope with while performing) you can improve your ability (mentally and physically) to cope. 

Athletes need to both familiarize themselves with heat (especially if performing in a hotter country) and train for it, they are two different things, and monitoring your temperature will let you know when you have achieved both!


A study from Cornell University found that employees are more accurate and productive in a work environment of 25 °C. At 20 °C the same employees were nearly half as less productive, and the number of errors in their work doubled. 

Warmer temperatures also foster trust and empathy between co-workers. 

As the external temperature drops, and your internal one follows, you will find it harder to think critically and perform mentally. Being too cold also impacts your mood and ability to pay attention to the environment. Have you ever considered it could be your internal temperature that is making it hard to concentrate? 

How to Monitor Body Temperature

You can go old fashioned with a thermometer, to measure your temperature manually and sample some results. This is time consuming and you’ll probably forget after your first day of enthusiasm. Digital devices, apps, and wearable thermometers are paving the way for a robust personal thermo data set that can give you the best picture of your own body heat!

Manual Measurement

  • Take four readings every day, for three days:
  • Three readings before eating each meal. 
  • One reading before you go to bed.
  • This will give you an average body temperature.
  • The ideal body temperature lies between approximately 36.7 °C and 37.2 °C. 
  • Note that your body temperature will increase as the day goes on.
  • Usually your temperature will increase 20 to 30 minutes after eating (by as much as as much as 2 °C), signalling that your metabolic rate has risen to aid digestion. 
  • If your body temperature doesn’t increase after eating you might have consumed a nutritionally empty meal.

Body Temperature Monitoring Technology

Note that the market for temperature measurement has grown hugely in the past two years due to additional thermo vigilance. But, not all devices are accurate. Theoretically measuring external temperature can be mapped to internal temperature, if the quality of the device is sufficient.

The CORE Body Temperature monitoring technology delivers athletes, coaches, and sports scientists with insights to help them monitor core body temperature in real-time and improve heat training, heat acclimation, optimise cooling strategies and use these insights for strategic racing.  CORE is the first wearable that “continuously and accurately measures core body temperature on the go.” This small, non-intrusive device is worn connected to a strap around your torso and integrates with your smartphone, Garmin, Apple Watch and other select wearables so you can view your body temperature data live and direct. CORE measures core body temperature with “medical grade accuracy,” a feature offered by few other products, making it one of the most accurate wearable temperature monitors currently available on the market.

The Sense Smartwatch from Fitbit takes skin temperature readings at night while you sleep for a minimum of 3 nights to get a baseline temperature, then it uses this data along with other metrics like heart rate to determine how stressed you are. A major drawback to the Fitbit Sense is that it only measures your skin temperature at night. Good for basal body temperature.

This Celsium Smart Wearable thermometer takes temperature readings every four seconds with a small sensor that attaches to your armpit and connects to your smartphone. With core temperature readings accurate to within +/- 0.2 °C, and has a much more wallet friendly price.

Thermal Guns and (cheap) Infrared Thermometers allow a temperature reading to be taken without physical contact, they have been utilized by many businesses to screen customers. However, these thermal guns are far less accurate than one might assume, with some having +/- 6 °C inaccuracy (rendering them essentially useless). It’s not uncommon to get a reading from a thermal gun that, if it were accurate, would mean you were dead. Knowing your actual body temperature could get you past some overzealous security guard who hasn’t calibrated his device.


Advancements in telemedicine and personal health tracking are rapidly increasing our ability to track personal temperature and other vital signs. By itself body temperature can highlight useful information about your metabolism, weight, sports performance, menstrual cycle and aid in the early detection of illness. 

In combination with the rest of your health data your temperature can reveal critical insights that help you heal quicker, stay focused longer, enhance performance and even outline your circadian rhythm. Basis Health supports real time temperature monitoring and automated insights that help you level up your health and performance.



13 mins read
The Ultimate Guide To Body Temperature

Body temperature is a vital, biomarker that can reveal deep insights into your current health.

In this article:

Article Highlights:

  • Why track body temperature?
  • Sports performance
  • Body Temperature Monitoring Technology