Body temperature is easy to measure and it can tell you a surprising amount about your health. It does take some patience - sometimes a week or two of measuring regularly before you get any data you can use - but it takes 30 seconds and it's free, so take a look at 4 ways you can use it to learn about your health. Your temperature can tell you...
1. How Fast Your Metabolism is Running
This is the one most people know: body temperature is a decently reliable guide to metabolic rate. When people lose weight, their body temperature drops as their metabolism slows down. This is why some people feel cold all the time when they’re dieting, especially if they’re doing something extreme.
For example, take the Biosphere 2 study. In this study, 8 healthy adults spent 2 years sealed off from the outside world. They had to eat a low-calorie diet that caused 18% weight loss in men and 10% weight loss in women. And these people weren’t overweight to begin with.
As the researchers running the study reported, subjects going in had an average body temperature around 98.6. But by the time they got out, their body temperatures had dropped to 96-97 F, sometimes lower. When they started eating again and regained weight, their body temperature went back up to normal. This mirrored their thyroid function – lower body temperatures corresponded to lower thyroid hormone levels. Basically, their body temperature dropped as their metabolic rate dropped, and then rebounded when their metabolism rebounded.
The same thing happened in the famous Minnesota Starvation Experiment, otherwise known as a really fantastic argument for how calorie restriction makes people crazy.
Intriguingly enough, there might actually be an upside to this whole process. Yes, it stinks that metabolic rate inevitably drops with weight loss – it’s harder to maintain weight loss because you’re always fighting your body’s desire to regain the weight. But on the bright side, the temperature reduction and metabolic slowdown may increase longevity.
Basically, your metabolism is a little like a candle. The hotter you burn it, the faster it burns out. Slow it down and you might be a little chilly, but the candle will burn longer.
How You Can Use This:
Keeping track of your body temperature can help you determine what kind of metabolic response your body is having to a particular diet. Track your temperature as you lose weight. If one thing tanks your metabolism, trying something else may help. For example, if a low-carb diet doesn't work for you, maybe moderate-carb will be better. Or vice versa! (You can get more ideas and information about metabolic slowdown and weight loss here and here).
2. When Your Body Wants to Go to Sleep (Your Circadian Rhythm)
Your circadian rhythm is the daily cycle of hormones that synchronizes your body’s internal clock to the real world. Circadian rhythms are why you get tired at night and wake up in the morning (or, if your circadian rhythms are totally out of whack, they’re why you don’t get tired until 5 am and then can’t wake up until noon). In healthy humans, temperature varies as part of the circadian cycle:
- Minimum body temperature = most sleepy (usually this is sometime in the very early morning). Typically, this is a couple hours after peak production of the hormone melatonin.
- Maximum body temperature = most awake (usually late afternoon).
As this study explains, morning people have an earlier temperature peak (corresponding to the time of day when you’re most awake), while evening people have a later peak. Typically, morning people are shifted about an hour earlier. Basically, your temperature cycle is a decent way to figure out when your body wants to be awake and asleep.
How You Can Use This
If you’re tired a lot, can’t get up in the morning, or can’t fall asleep at night, use your body temperature to see how your circadian rhythms are doing. Is your peak temperature really early or really late? If your temperature is peaking at 11 pm and not reaching its low until 8am, well there's your problem! And here’s the really good news: if you find something off, you can change your body temperature cycle using bright lights.
- Use bright lights in the morning if you need to start waking up earlier.
- Use brighter lights in the afternoon or evening if you need to go to bed later (this study has some more on that, if you're interested)
3. How Stressed Out You Are
In humans (and rats), body temperature rises with stress. Technically, this is called stress-induced hyperthermia.
This has mostly been studied in acute stress, which is actually the least useful to everyday life because if you’re under acute stress, you know it. If you’re sweating over an exam, you don’t need a thermometer to tell you that you’re stressed out. But some studies, like this one and this one, have also fund a chronic temperature increase with long-term stress.
How You Can Use This
Take your own temperature every day for a week or so when you’re not stressed. Do it at the same time every day, so you don’t get thrown off by the circadian cycles. That gives you a baseline. Then you can get a rough idea of whether or not you’re stressed by noticing an increase in temperature. Temperature alone isn’t a totally reliable guide, but think of it as one potential sign of a problem, especially if it’s combined with…
- Digestive pain, constipation, and/or diarrhea
- Sleep problems/insomnia
- Cravings for sugar and simple carbs
4. Whether Your Metabolic Health is Improving
(This may be a little confusing, so just to clarify: "Metabolic rate" above refers to how many calories you burn in one day just by existing. "Metabolic health" refers to things like carb tolerance, insulin sensitivity, and blood lipids.
People with good metabolic health have more variation in daily temperature: their highs are higher and their lows are lower. They also have more consistency from day to day.
This study found that metabolically healthy have a spikier circadian temperature rhythm (bigger differences between the low and the high point) than inactive people. The study compared people with no metabolic problems to people with at least one symptom of metabolic syndrome (blood sugar problems, large waist circumference, high blood pressure, low HDL cholesterol, and/or high triglycerides). The healthy group had a typical temperature variation of about 1.5 degrees C (around 3 degrees F), while the unhealthy group varied less than 1 degree C (around 2 degrees F).
How You Can Use This
Measure your temperature at several points during the day to find your typical daily variation between your high and your low. Do it for a few days just in case something was weird the first time. Then you can measure again to track whether/how much your metabolic health is improving.
If you’re not getting the results you want, other studies of body temperature suggest that exercise is a good way to change that. The study above found that a major predictive factor of body temperature amplitude was physical activity. That makes perfect sense, since physical activity is one of the very best ways to improve metabolic health. So physically active people are also metabolically healthy people: it’s the same group. This study confirmed that active people had a spikier temperature rhythm, with a bigger gap between the overnight low and the afternoon high.
Summing it Up
Body temperature is easy to measure: no blood glucose meters, no skin pricking, no blood or needles. And if you’re interested in your metabolic rate, stress levels, circadian rhythm, or overall metabolic health/physical fitness, body temperature is a relatively fast and easy way to track those things.
Not everyone's "normal" body temperature is exactly 98.6. The points above are mostly about changes from baseline, not absolute numbers - figure out your own typical temperature and go from there.
Some other technique notes:
- Because temperature naturally cycles over the day, measure at the same time every day.
- For women: your body temperature will naturally rise about 1 degree during the luteal phase of your menstrual cycle (starting 14 days after your period begins and lasting until your period starts again). So make sure to account for that in your measurements.
Get a good thermometer and remember: some of these changes are 1-2 degrees on average, so watch for a consistent pattern of smaller changes, not huge dramatic jumps.