The Four Windows of Your Day
Core body temperature isn't a fever check but a continuous rhythm marker, and once you can see its shape, your day splits into four windows — rise, peak, landing, night — each with its own best use.
Body temperature is the most honest rhythm marker we have. Vire co-founder Antti Immonen explains what it is actually telling you, and how every day divides into four metabolic seasons.
The Vire Clip exists because of my daughter.
When Emilia was born, we were told by the doctors to hope for the best, but expect the worst.
At home, I wanted to track her vital signs and core body temperature. Not once, but continuously through the night. The most reliable place to read the core of a very small baby turned out to be the waistband of her diaper.
That constraint became our form factor, and the question behind it became the company: what can we learn if we stop sampling the extremities and sit close to the core, all the time?
What we found there was not a number. It was a rhythm.
Temperature is not a flat reading
Most of us were taught exactly one thing about body temperature: 37 °C is normal, and anything above it means you are ill. That is a fever thermometer's view of the world, and it is a little like believing the earth is flat.
Your temperature moves all day. Think of it as a receipt. Everything your metabolism does produces heat, so the core temperature you carry around is the running sum of your hormones, your meals, your movement, your stress and your sleep. Eat protein and up to a third of its calories leave again as heat. Exercise and about 80% of what you burn becomes heat rather than useful work. Ovulate, and progesterone lifts the whole waveform.
We all have a distinct rhythm to the movement of our own temperature. This is the rough scale we move through on a daily basis. So treat it as a starting map rather than a verdict.
Turn your phone sideways to see this scale full width.
Below 36.8 °C you are in night metabolism. Above 37.0 °C you are in the day. Near maximum heart rate many people pass 39 °C, and that is not a fever. It is a working body.
Your body is a prediction machine
The reason rhythm matters more than any single reading is something called predictive homeostasisPredictive homeostasis
Adjusting the body's internal state ahead of a predictable event rather than reacting once it happens. Your circadian clock knows roughly when you wake, eat, move and sleep, so hormones, temperature and metabolism start shifting before the event arrives. Keep the timings regular and the body can prepare. Scramble them and it has to improvise.. Your body does not wait for events and then react to them. It prepares. The bigger the change it expects — and waking, eating, training and sleeping are the big four — the more it wants advance notice.
Give it that notice and it can pre-load the right hormones for what is coming. Withhold it and the body improvises: cortisol at midnight, digestion that arrives late, sleep that never quite settles.
Which brings us to the part of this I find genuinely useful in daily life.
Four metabolic seasons, every day
Once you can see the curve, a day stops being a flat block of hours. It divides into four windows. I think of them as seasons: the body ignites, runs hot, lands, and recovers. They are not clock times, they are your times, and the Vire app tunes to them automatically. The useful question is not what the textbook says, but whether your own timings feel right for your body.
Turn your phone sideways. This is the centrepiece of the article and it has a whole day to fit in.
Tap a window to explore it, or use the buttons above.
A representative day. Your own windows shift with your sleep and wake times, and the afternoon dip inside the peak is normal. It works well for a short nap.
The window most of us get wrong is the landing.
You cannot flip a hormonal context by switching off a light
In humans, core temperature reliably begins falling about two hours before sleep onset, and the first non-REM sleepNon-REM sleep
Everything that is not dreaming sleep, including the deepest and most restorative stages. It is easy to assume REM is the good stuff and the rest is filler. It is closer to the opposite: non-REM is when most physical repair happens, and it is also when the body does its heaviest cooling. episode is most likely to arrive at the steepest point of that decline.1 The descent is not a consequence of sleep. It is the runway.
And here is the detail that changed how I think about evenings. Falling asleep quickly turns out to depend less on core temperature itself than on heat actually leaving the body through the hands and feet. In a controlled study, the distal-to-proximal skin temperature gradientDistal-to-proximal gradient
The difference between how warm your hands and feet are and how warm your torso is. When the vessels in your extremities open, they act like radiators: the hands and feet warm up while the core cools down. So this gradient is really a measure of how fast heat is leaving your body. predicted how fast people fell asleep better than core temperature did, better than its rate of change, better than heart rate, melatonin onset or how sleepy people said they felt.2
That is why a warm shower before bed helps rather than hurts: you warm the skin, the vessels in your hands and feet open, and the core sheds heat faster. It is also why late hard training is such an effective way to ruin a night. You spend the landing window pushing temperature up when your physiology is trying to bring it down, and a high temperature at sleep onset shows up later as less deep sleep.
Once you are asleep, the shape keeps talking. Your nightly minimum should sit roughly two thirds of the way through your sleep, and the trough should be wide rather than a brief dip.
Turn your phone sideways to see the whole night.
Squint past the small ups and downs and look for the trend: a wide, low trough with its minimum about two thirds of the way through the night.
Your body wants you to be boring
None of this needs to be perfect. What it needs is to be repeated.
If the majority of your days carry a clear rhythm, your body can stop guessing and start preparing, and it will get the right things to the right places at the right times.
The two timings that matter most are the edges: when you rise and when you land. Those are the moments your system uses to set its clock, so keeping them steady buys you more than optimising anything in the middle.
I should be honest about the limits too. Measuring core temperature well is genuinely hard, which is most of the reason it has stayed a fever signal for so long.
When you have a fever your physiology pushes heat to the surface and almost any thermometer will find it. When you are healthy, temperature hides.
If a reading tells you that you sit below 36 °C, be sceptical: it is usually the measurement, not the metabolism.
That difficulty is also why continuous, close-to-the-core measurement is now being taken seriously well beyond our own field, from sleep research to studies of how people cope with hotter summers.3
And I should stay humble about the rest of it. Continuous core temperature is a young signal, and most of what I have written here is generalised guidance rather than settled fact.
I do not know yet which of these shapes will hold up across thousands of real days, whose bodies will politely ignore my rules of thumb, or what else is sitting in this curve that none of us has thought to look for. That, to me, is the interesting part.
So: look at your own four windows this week. Do the timings feel right for your body? Tell us what you see, and just as usefully, what you do not.
Emilia is now five, and thriving.
Antti Immonen is a co-founder of Vire Health. This piece draws on his sessions with the Vire beta community in July and August 2026.
1. Harding EC, Franks NP, Wisden W. The Temperature Dependence of Sleep. Front Neurosci. 2019;13:336.
2. Kräuchi K, Cajochen C, Werth E, Wirz-Justice A. Functional link between distal vasodilation and sleep-onset latency? Am J Physiol Regul Integr Comp Physiol. 2000;278:R741–R748.
3. Koch M, Matzke I, Huhn S, et al. Wearables for Measuring Health Effects of Climate Change–Induced Weather Extremes: Scoping Review. JMIR Mhealth Uhealth. 2022;10(9):e39532.