5 Signals Your Body Sends You Before It Gets Sick

By the time the sore throat turns up, your body has usually been sending you signals for days. Some of those warning shots you'll recognise straight away — the flat energy, the reluctance to leave the house, the fuzzy feeling in your head. Others are subtle enough to sail past you completely, because they tend to show up in subtler ways.

For years, the idea that you can sense illness coming was written off as hindsight — you only remember the odd Tuesday because you woke up ill on Thursday. Then hundreds of thousands of people started wearing rings and watches logging heart rate, temperature, and sleep every night, and researchers went looking into the data. And it showed them something very interesting. We were on to something after all.

Now, with flu season just around the corner, it's important to be able to notice those signals before you actually get sick. Here are five signs something might be brewing, starting with the ones you can catch yourself, plus some of our favourite products to support your body while it fights against flu and cold.

The ones you'll probably notice

Your appetite and your interest in people both drop. Low appetite, fogginess, and the urge to cancel plans are part of what researchers call sickness behaviour — a coordinated response driven by inflammatory signalling to the brain, mapped by Robert Dantzer and colleagues in Nature Reviews Neuroscience. It's the immune system redirecting energy, not a character flaw.

Sleep turns patchy for no obvious reason. Broken nights work in both directions here — a sign, and a risk factor. Aric Prather's team at UCSF tracked 164 people's sleep for a week, then exposed them to a cold virus. Those averaging under six hours a night were 4.2 times more likely to come down with it.

The ones you'd miss without data

Your resting heart rate creeps up. In a Stanford study of smartwatch data from nearly 5,300 people, published in Nature Biomedical Engineering, raised resting heart rate appeared roughly four days before symptoms in COVID-positive participants — and in 22 of the 25 cases with symptom dates, it showed up at or before the first symptom.

Your HRV dips. Heart rate variability tends to fall when the body is carrying load. In Mount Sinai's Warrior Watch study, Apple Watch HRV changed up to seven days before a positive swab, then settled back within a fortnight. As Dr Jo Brown, head of human performance at Nurosym, told The Healf Source, "HRV reflects regulatory capacity, which is the ability of the autonomic nervous system to dynamically respond to load." Fighting something off uses up some of that capacity.

Your temperature shifts before you feel feverish. The UCSF and UC San Diego TemPredict study tracked continuous finger temperature in Oura ring wearers and found changes a mean of three days ahead of reported symptoms — small moves, nowhere near what you'd call a fever.

Where the evidence gets murky

Nearly all the wearable research came out of the pandemic, so it's built around one virus, and the numbers are less tidy than the headlines. In the Scripps DETECT study in Nature Medicine, resting heart rate alone barely separated infected people from the worried well — only 30% of symptomatic positive cases ran more than two standard deviations above their own baseline. Sleep and activity data performed better.

Baselines drift for dull reasons too: a late meal, a glass of wine, a hard session, an overheated bedroom. Sleep scientist Dr Sophie Bostock put it plainly in The Healf Source: "Sleep varies from night to night, and that is normal."

What to do with an odd few days

If you're experiencing two or three of these symptoms together along with the sense that something's just off, it's worth acting on: be gentler with your movement instead of pushing through, get to bed early while it's still your choice, eat properly, drink more than feels necessary. Then close the app. The aim is a lighter week, not a diagnosis.

Things you can do to protect yourself

These signals don't call for a dramatic response. They simply tell you that your body might need a bit of support while it's fighting off whatever seems to have entered your system. These following products from BioCare, a UK brand with more than 35 years of formulation behind it, can help support your systems in the interim.

Vitamin D3 + K2

From roughly October to early March, the sun in the UK sits too low and is out for too little time for your skin to make adequate vitamin D — which is why the NHS advises everyone to consider a daily 10 microgram supplement through autumn and winter. Vitamin D contributes to the normal function of the immune system, and K2 is there for bone maintenance and normal blood clotting. BioCare's version is a liquid, so you can drop it straight under the tongue or into water and adjust the dose rather than committing to a fixed tablet.

Magnesium glycinate for the patchy-sleep stretch

Magnesium is bound to glycine here, which makes it gentler on the stomach than the cheaper oxide forms, which is especially important if you've tried magnesium before and gave up on it. Each BioCare capsule delivers 100mg of elemental magnesium, and magnesium contributes to normal nervous system function, normal psychological function, and helps reduce tiredness and fatigue. It's a sensible one to keep in the cupboard for the weeks when your sleep goes sideways and your energy follows.

Immune defence, for the first-sign days

BioCare's Immune Complex is the one to reach for when two or three of the signals above start appearing. It layers vitamin C, vitamin D3, and zinc — all of which contribute to the normal function of the immune system. Vitamin C and zinc also contribute to the protection of cells from oxidative stress. Take two capsules a day with food, across the stretch when your immune system is struggling.

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