Your body’s like a little furnace. It puts out heat all the time. It comes from your body doing the work that keeps you alive. When it puts out a lot more or a lot less heat than usual, it’s trying to tell you there’s a problem.
Normal Range
Not everyone’s “normal” body temperature is the same. Yours could be a whole degree different than someone else’s. A German doctor in the 19th century set the standard at 98.6 F, but more recent studies say the baseline for most people is closer to 98.2 F.
For a typical adult, body temperature can be anywhere from 97 F to 99 F. Babies and children have a little higher range: 97.9 F to 100.4 F.
Your temperature doesn’t stay the same all day, and it will vary throughout your lifetime, too. Some things that cause your temperature to move around during the day include:
- How active you are
- What time of day it is
- Your age
- Your sex
- What you’ve eaten or had to drink
- Where you are in your menstrual cycle
Your temperature reading changes based on where on your body you measure it. Underarm readings can be a degree lower than what you’d find from your mouth. Rectal temperatures usually are up to a degree higher than mouth readings.
A body temperature higher than your normal range is a fever. It’s hypothermia when the body temperature dips too low. Both need to be watched.
Fever
How high is too high when it comes to your temperature? Anything above 100.4 F is considered a fever. You may feel terrible, but on the whole, a fever isn’t bad for you. It’s a sign your body is doing what it should when germs invade. It’s fighting them off.
However, if your temperature is 103 F or higher or if you’ve had a fever for more than 3 days, call your doctor. Also call if you have a fever with symptoms like severe throat swelling, vomiting, headache, chest pain, stiff neck or rash.
For children, fevers are a bit more complicated. Call your pediatrician if your child is:
- Under 3 months and has a rectal temperatures of 100.4 F or higher
- Between 3 months and 3 years and has a rectal temperature over 102 F
- Older than 3 years and has an oral temperatures above 103 F
- Between 3 and 6 months and -- along with a fever -- is fussier or more uncomfortable than usual, or doesn’t seem alert
- Sick enough for you to be concerned, regardless of what the thermometer says
If your body loses too much heat, it can be very serious, even fatal. Hypothermia is when your body temperature goes below 95 F. You might think of hypothermia as something that only happens when you’re exposed to extremely cold weather for a long time. But it happens indoors, too.Hypothermia
Hypothermia is a special concern for newborns and the elderly.
Babies may not be good at regulating their temperature. They can lose heat quickly. It’s important to keep them warm. A temperature below 97 F is considered too low for babies.
Older adults can also struggle to keep their body temperature in a normal range if they’re somewhere with intense air conditioning or there’s not enough heat.
For both the elderly and young children, a below-normal body temperature can be a sign they’re sick.
Other things can also make you more likely to get hypothermia. They include:
- Alcohol or drug use
- Hypothyroidism (an underactive thyroid)
- Anorexia
- Stroke
- Sepsis (overwhelming infection)
- Parkinson’s disease
- Nerve damage
- Malnutrition
- Medicines such as antidepressants, antipsychotics, or sedatives
- Anesthesia
SOURCES: Kids Health: “Fever and Taking Your Child’s Temperature.” Mayo Clinic: “Fever: First Aid,” “Fever,” “Hypothermia.” Paediatrics & Child’s Health: “How to take a child’s temperature.”Show Sources
University of Rochester Medical Center: “Vital Signs (Body Temperature, Pulse Rate, Respiration Rate, Blood Pressure)."
Your body has an internal thermostat and it aims to maintain your temperature as close to your goal as possible. The stability of that number, known as your core body temperature, reflects your body’s ability to turn up the heat or cool things down to keep you within your ideal range.
Each day, your body has plans to adjust your temperature to follow your circadian rhythm by warming you up to start the day and cooling you down for sleep. Yet diet, exercise, hormones, and many other things affect your temperature. As a result, your body is constantly making adjustments to keep it all balanced:
- When you are too hot, your body tries to radiate, or get rid of heat, by widening your blood vessels to carry excess heat to your skin’s surface. As sweat evaporates off your skin and blood loses heat to the air, your body cools.
- When you are too cold, your body tries to insulate, or trap heat by narrowing your skin blood vessels so the blood keeps more heat in your core, as well as raising your temperature through shivering.
The stability of your temperature is an indicator of your body’s ability to adjust in the face of challenges. It signals how you’re recovering, how prepared you are for the day, if you’re coming down with an illness, or if hormones in your body are hard at work.
Your Temperature Naturally Fluctuates
On average, an individual’s core body temperature varies from 36.5–37.5 °C (97.7–99.5 °F) over the course of a day. Your body temperature doesn’t remain constant, however, it fluctuates according to your circadian rhythm.
Generally, this means your body temperature is at its lowest a few hours before you wake and its highest an hour or two before bed.
An individual’s core body temperature typically changes by about 1 °C (1.8 °F) between its highest and lowest points each day. Anything outside of that range indicates that something is challenging your body and preventing it from adjusting.
At the surface of your skin, where heat from your blood and cold from the environment are more intermingled, the range is typically much larger.
Your skin temperature and core temperature can even change in opposite directions throughout the day. For example, if your core temperature is too high, your body tries to cool itself by pushing blood to the skin so it can shed that excess heat. But in the case of fever, or after ovulation, both will rise similarly.
With all of these changes, it’s important to keep in mind where you’re measuring from before you compare numbers.
Which Patterns Matter?
During the day, your temperature varies as you move, eat, drink, socialize, and change your environment. Because there is so much variation in temperature throughout the day, if you want to measure your overall health status, it’s best to measure your temperature at night when your body is stable.
At night, your body’s temperature is a reflection of what happened during the day—is your body stressed outside of its ideal range or is it able to maintain your stable thermostat?
If your body temperature is outside of your normal range, your body is signaling to you that something is pushing it beyond what it can adjust for.
Look for these patterns in your nightly core temperature:
Disrupted Circadian Rhythms
Your circadian rhythm helps your body move between being awake or asleep. As part of this rhythm, your core body temperature drops near your optimal bedtime and rises again just before you wake up. Keep in mind that melatonin release helps trigger your body’s cooling process and may be easily disrupted by blue light.
If your average temperature is significantly higher or lower than average, or if it fails to change when you try to go to sleep, then your body may be stuck under strain—preventing it from flexibly adjusting throughout the night. If you fall asleep at a different time than you normally do (e.g., traveling to a new time zone) your temperature rhythms may also be misaligned.
Potential Illnesses
If you’re getting sick, even before you get a fever, your body’s temperature may rise. While estimates vary, normal daily temperature fluctuation is around 1 °C (1.8 °F). If you spot a high temperature outside of that range, it may mean your body is fighting to keep you healthy. Some illnesses may even cause a fever. Regardless, a positive sign of recovery is your temperature returning to normal afterward.
Internal Heat
Powering your organs can generate a lot of internal heat. If you have a large meal or a few drinks close to bed, you may find that your temperature remains elevated throughout the night. This is evidence that your digestive system is working overtime.
You may spot changes around 0.5 °C (0.9 °F). Being in touch with your body can help you identify if overheating is due to something you can control (e.g., mealtimes or bedding) or is the result of another bodily change (e.g., hot flashes or melatonin).
Menstrual Cycles
If you track your menstrual cycle, you may see your temperature range shift with your hormones. This usually results in lower body temperatures during the first half of the menstrual cycle (follicular phase), followed by a rise in the second half (luteal phase). Note that using hormonal birth control may suppress monthly temperature variation and/or increase your average temperature.
Things to Keep in Mind
You have the power to influence your body temperature more than you might think. There are even Tibetan monks who meditate and raise their skin temperature high enough to dry a wet towel resting on their backs. We recommend starting with some of these simpler hacks:
- A hot shower can encourage your body to rapidly cool itself afterward, signaling to your internal thermostat “It’s time for bed.”
- A cold shower can stimulate your system and cue your body to heat itself to stay awake.
- A cool room can lighten your body’s workload by making it easier to maintain an ideal temperature during sleep. Regardless of your temperature preferences, physiology and science both point to the ideal nighttime room temperature being around 65 degrees Fahrenheit (18 degrees Celsius).
Observing your body’s day-to-day temperature can inform you. Start monitoring your trends and see what patterns emerge.