Training Through the Heat Without Digging a Recovery Hole

Over the last two days we took apart the summer HRV panic: the number drops in the heat because thermoregulation is an autonomic load that wears the same face as fatigue, and you separate the two by reading the trend and the context, not the morning value. That leaves the question that actually matters when you're staring at a low reading and a hot forecast: what do I do today? It starts with a reframe most people miss. That low HRV isn't just a tax on your training — it's the receipt for an adaptation you'd pay good money for.

The Reframe: Heat Is a Stimulus, Not Just a Stressor

We treat altitude camps as a badge of serious training and heat as something to be endured. The physiology says that's backwards. Lorenzo, Minson and colleagues, in a now-classic 2010 study in the Journal of Applied Physiology, put trained cyclists through ten days of heat acclimation and measured the payoff in cool conditions: plasma volume up about 6.5%, VO2max up 5% in the cool (8% in the heat), and time-trial performance up 6% (8% in the heat). Those are altitude-camp numbers, bought with a heat wave you were going to live through anyway.

The mechanism is the same blood-volume story that underlies a lot of endurance adaptation. Périard, Racinais and Sawka's 2015 review in the Scandinavian Journal of Medicine & Science in Sports lays out the full suite: repeated heat exposure drives plasma volume expansion, earlier and heavier sweating, lower sweat sodium, and lower core and skin temperatures, so that over a week or two your exercise heart rate at a given effort settles back down. A bigger, more dilute blood volume is better at both cooling you and delivering oxygen. That's why the gains show up in the cool.

So the early-week HRV crash deserves a different label. It's not the heat damaging you; it's the front end of an overload your body is in the middle of absorbing — heavy now, valuable later, exactly like the first week of any hard block.

Let the Trend Make the Call

That reframe doesn't mean ignore the number. It means use it the way yesterday's piece described — as a trend, with context — to decide between two moves.

Hold the plan and push when the rolling average is flat or already clawing back up, and the dips track the weather rather than a steady descent. This is the acclimation window Flouris and colleagues documented in their 2014 work in the European Journal of Applied Physiology: HRV suppressed early in heat exposure, then recovering as adaptation sets in. A rebound mid-heat-wave is your green light. Keep going and you bank the plasma volume.

Deload when the rolling average is in a genuine multi-week decline with widening day-to-day variability — the non-functional-overreaching signature from Plews and colleagues, where the coefficient of variation climbs as the system destabilizes. That pattern doesn't bounce on a cool night, and pushing through it buys you nothing but a hole.

The trap is doing the opposite of each: deloading through a normal acclimation dip and throwing away the adaptation, or grinding through real overreaching because "it's probably just the heat." The trend tells you which room you're in.

Autoregulate Intensity, Not Just Volume

When a day genuinely reads low-readiness, the instinct is to skip. Usually the better move is to keep the session but pull the intensity down. Heat acclimation is driven largely by time at an elevated core temperature, and you bank that just as well in a relaxed aerobic session in the warmth as in a brutal one — without the neuromuscular and glycogen cost that would actually deepen fatigue. So on a low day in a hot block, trade the intervals for steady aerobic work. You keep feeding the adaptation while letting the training stress recede. That's the whole logic of autoregulation: match today's load to today's readiness, and let easy days still do useful work.

Defend Everything That Isn't Training

Two of the biggest levers in a hot block aren't workouts at all.

The first is hydration. Dehydration shrinks the very plasma volume you're trying to expand, and — as Carter, Cheuvront and colleagues showed in 2005 — it independently depresses HRV after exercise heat stress. Replacing fluid and electrolytes through and after hot sessions protects both the adaptation and the recovery signal you're using to navigate.

The second is sleep temperature, and it's badly underrated. O'Connor and colleagues' 2025 study in BMC Medicine found that a bedroom above ~24°C is enough on its own to depress sleeping HRV and lift overnight heart rate. A hot bedroom is a double loss: it sabotages the recovery itself and it corrupts the number you read in the morning. Cooling the room — a fan, a cracked window, lighter bedding, an earlier pre-sleep shower — is one of the highest-leverage moves available, and unlike most training tweaks it's nearly free.

Give It the Timeline

Acclimation isn't instant. The literature converges on roughly seven to fourteen heat sessions to capture most of the adaptation, with the autonomic markers rebounding across that window — and Flouris and colleagues' decay data is the reminder that it's a use-it-or-lose-it bank, fading within about two weeks once the heat exposure stops. So plan it as a block, not a single brave session. Expect the early suppression, hold the plan through it as long as the trend cooperates, and re-baseline your HRV once you've acclimated, because your "normal" will have shifted.

This is the problem Dorsi is built to handle — a readiness number that's down for the right reason, in the middle of an adaptation worth finishing. The system that reads the trend and the context, instead of flinching at a single hot morning, is the one that lets you train the heat on purpose. The weather will pass. The plasma volume you built will outlast it. Let the trend, not the thermometer, decide when to back off.

Sources

The performance and plasma-volume gains from a ten-day heat acclimation block — measured in cool conditions — come from Lorenzo, Halliwill, Sawka and Minson (2010), "Heat acclimation improves exercise performance," in the Journal of Applied Physiology. The mechanistic account of heat acclimation (plasma volume expansion, earlier and heavier sweating, lower sweat sodium, reduced core and skin temperature, lower exercise heart rate) draws on Périard, Racinais and Sawka's 2015 review in the Scandinavian Journal of Medicine & Science in Sports. The HRV rebound during acclimation and its decay within roughly two weeks are from Flouris and colleagues (2014) in the European Journal of Applied Physiology. The non-functional-overreaching signature — a declining rolling average with rising coefficient of variation — is from Plews and colleagues' 2012 case comparison in the same journal. The independent effect of dehydration on post-heat-stress HRV is from Carter, Cheuvront and colleagues (2005) in the Journal of Thermal Biology, and the effect of warm bedrooms on sleeping HRV is from O'Connor and colleagues' 2025 study in BMC Medicine.