WHOOP can't diagnose anything, but the signals it records overnight (resting heart rate, HRV, respiratory rate and skin temperature) often shift when your body is fighting something, sometimes before you feel ill. Research on wearables backs that up, with big caveats. The useful pattern is several signals drifting away from your own normal at the same time, not one odd number.
You wake up feeling fine, but your recovery is yellow, your resting heart rate is a few beats higher than usual and your skin temperature has a little orange warning next to it. Two days later you have a sore throat. Looking back, the data seemed to know first.
That experience is common enough that "can WHOOP tell you're getting sick?" comes up again and again in the WHOOP subreddit (Reddit). The honest answer is: sometimes, partly, and only if you know what to look for.
What your strap is listening to overnight
WHOOP's Health Monitor shows the metrics that matter most here, all taken from the previous night's sleep: respiratory rate, resting heart rate, HRV, blood oxygen and skin temperature. Each one is marked as inside or outside your personal typical range, and WHOOP suggests that if one is outside it, you keep an eye on it over the next few days (WHOOP).
Sleep is a good time to measure, because you're still and the usual daytime noise is gone. In a WHOOP study published in PLOS ONE, the authors describe resting heart rate and HRV being taken during slow-wave sleep and respiratory rate being taken across the night, and they found nightly respiratory rate to be remarkably stable within a person (Miller et al., 2020). A stable signal is exactly the kind that makes a change easy to spot.
WHOOP's own guidance on what tends to happen when you get sick is simple: resting heart rate tends to rise and HRV tends to fall, and knowing your baseline can help you notice it, sometimes before you feel sick (WHOOP).
What the research actually found
Several studies have looked at whether wearable data shifts before symptoms appear. The results are encouraging, but none of them turn a wearable into a diagnostic test.
- Heart rate, steps and sleep. A Stanford-led study of smartwatch data found that 26 of 32 people with COVID-19 (81%) had changes in heart rate, steps or sleep. Of the 25 cases with enough data, 22 were flagged at or before symptom onset, with a median of four days before symptoms. The same alerts also fired during stress, travel, alcohol and holidays, and the method couldn't tell one illness from another (Mishra et al., 2020).
- Respiratory rate. In the WHOOP study mentioned above, a model built on overnight respiratory rate and other metrics identified 20% of COVID-positive members in the two days before symptoms began and 80% by the third day of symptoms (Miller et al., 2020). The authors include WHOOP researchers.
- Skin temperature. A study of Oura ring users who reported COVID-19 found that finger temperature rose during illness, and 38 of 50 people had "fever-like" days before they reported symptoms. The authors also found that temperature varies so much from person to person that a single cut-off for everyone wouldn't work (Smarr et al., 2020).
- Colds and flu. In a study where volunteers were deliberately exposed to flu or a common cold virus, models using wristband data (heart rate, skin temperature and movement among them) told infected from uninfected people with up to 92% accuracy for flu and 88% for the cold virus, and could predict who would become mildly or moderately ill about a day before symptoms began (Grzesiak et al., 2021).
These are small studies, several of them focused on one virus, and some were funded by or written with wearable companies. Read them as "the signals often move", not as "your strap will catch it".
Why your normal matters more than "normal"
A resting heart rate of 58 means nothing on its own. For one person it's a typical morning, and for another it's six beats above their usual. That's why WHOOP describes the value of its always-on data as building a baseline that can show when a metric is out of your normal range, even if it would look normal for the general population (WHOOP).
(We go into what a normal range looks like for HRV in what is a good HRV on WHOOP.)
Skin temperature: useful, but easy to fool
Skin temperature gets a lot of attention, and it deserves some caution. WHOOP builds your skin temperature baseline from your previous 90 nights, and is clear that it is not the same as core body temperature (WHOOP).
A high reading can mean fever or illness, but it can also mean a warm room, an extra blanket or a late workout. For people who menstruate, skin temperature usually rises after ovulation too. WHOOP's view is that a sustained rise above your baseline can be an early sign that your body is fighting something, especially when your respiratory rate shifts at the same time (WHOOP).
So before you read anything into a warm night, ask the boring questions first. Was the heating on? Did you sleep under a heavier duvet? Did you train late?
One signal is noise, several is a pattern
The most useful idea in all of this research is that illness rarely moves just one number. In the Oura study, the temperature spikes that lined up with illness tended to come with a higher heart rate, lower HRV and a higher respiratory rate at the same time (Smarr et al., 2020). Everyday disruptions usually leave a different fingerprint:
- Alcohol tends to push HRV down and resting heart rate up for a night or two, even when you feel fine (WHOOP). We cover that in why your WHOOP recovery can be low after a good night's sleep. For how long it can last, see how long alcohol affects WHOOP recovery.
- Hard training can lower HRV for 24 to 48 hours or more, depending on intensity (Stanley et al., 2013), and it should settle when you back off. If it doesn't, that's a load question, covered in when to deload using 7- vs 28-day strain and HRV trends.
- A warm bedroom or extra blankets can raise skin temperature by themselves (WHOOP).
When resting heart rate, HRV, respiratory rate and skin temperature all drift the wrong way for two or three mornings in a row, and you haven't been drinking or training hard, that's the combination worth paying attention to. It's also the kind of change a recovery forecast can't see coming, as we explain in can you predict tomorrow's WHOOP recovery.
What to do when your numbers drift
None of this tells you what's wrong. It just tells you something is different. A sensible response is conservative and low-cost:
- Check the obvious causes. Drinks, late meals, a hard session, travel, a hot room or your cycle. A shifting sleep schedule can pull HRV down too, as we explain in WHOOP sleep consistency and recovery.
- Look at the trend, not the day. One odd morning is common. Several mornings of drift across more than one signal is the pattern to take seriously.
- Ease off for a day or two. Swapping a hard session for something easy costs you very little if it turns out to be nothing. Here's what strain to aim for on WHOOP when your recovery is down.
- Pay attention to how you feel. If symptoms appear, or you're worried, talk to a qualified professional. Your strap's data is something to share with them, not a replacement for them.
If you do end up getting sick, the next question is when to train again. We cover that in when can you train again after being sick.
Frequently asked questions
No wearable can diagnose an illness, and WHOOP says its products aren't medical devices. What WHOOP can do is show when your overnight signals move outside your usual range, which research suggests often happens around the time you get sick and sometimes before symptoms appear.
It varies by person and by illness. In published studies, resting heart rate, HRV, respiratory rate and skin temperature have all shifted around illness onset. The most reliable sign is several of them moving together, compared with your own baseline.
It could be a sign your body is fighting something, but it's often just a warm room, extra bedding, a late workout or the luteal phase of the menstrual cycle. A sustained rise, especially alongside a change in respiratory rate, is more meaningful than a single warm night.
Not on its own. Red recovery is usually driven by a drop in HRV, and training, alcohol, stress and poor sleep can all cause that. If red mornings come with a higher resting heart rate, higher respiratory rate and warmer skin temperature, that's more worth watching.
This article is general information, not medical advice. If something feels off, talk to a qualified professional.
Catch the drift early
The hard part isn't seeing one number move. It's noticing when several of them have been drifting for a few days, while you still feel fine and are tempted to train through it.
That's one of the reasons we built ZONAL, a free web app in beta that works with WHOOP through a read-only WHOOP sign-in. It gives you a daily train or ease-off call, a recovery forecast for tomorrow, and Signal watch, which looks for early illness signs using skin temperature alongside resting heart rate, HRV, breathing rate and blood oxygen. Every morning it compares those signals with your healthy normal, and when several drift at once, it flags it so you can take it easy early.
ZONAL started after one of our founders had strep throat and wanted to see what their data had been doing in the days before, and to know when it was safe to push again. If you'd like a second pair of eyes on your mornings, you can try ZONAL free while it's in beta.
ZONAL is an independent app that works with WHOOP through a read-only WHOOP sign-in. It is not affiliated with, endorsed by, or sponsored by WHOOP, Inc. WHOOP is a trademark of WHOOP, Inc.
