The first warm weeks of spring often feel like regression. Your paces drop. Your heart rate climbs. You sweat more. You feel like you're losing fitness when nothing has actually changed.
What you're experiencing is adaptation in progress. And if you use it intelligently, it can make you 5 to 8% faster in 10 to 14 days.
What Heat Does to Your Body
When you run in high temperatures, your body simultaneously manages two competing demands: supplying your muscles with oxygen AND regulating your core temperature. Blood flow splits between active muscles and skin (for heat dissipation). Result: less oxygen available for muscles, so your paces drop at the same perceived effort.
But with repeated exposure, your body adapts. It increases plasma blood volume, allowing it to better fuel muscles AND dissipate heat simultaneously. It improves its ability to start sweating earlier and more efficiently. And it optimizes use of available oxygen.
Measurable Gains After 10-14 Days
Studies show 5 to 8% improvements in VO2max in hot conditions after just 10 to 14 days of regular heat exposure. Even more interesting: these gains partially transfer to cool conditions. Heat-acclimated runners show 5 to 6% performance improvement even in cooler weather. The mechanism: increased plasma volume persists regardless of temperature, meaning a more efficient cardiac output in all conditions.
How to Acclimatize Properly
Acclimatization doesn't happen by accident. It takes deliberate planning. The most effective acclimatization runs last 40 to 60 minutes at easy pace (around 60% of VO2max) in warm conditions. Shorter runs at moderate intensity also work but require more time for the same adaptations.
Key rule: don't train at high intensity in the heat at the start. Acclimatization must happen at moderate intensity. Intense efforts in high heat without prior adaptation risk heat illness and degrade training quality.
Standard protocol: 10 to 14 days of daily or near-daily runs in warm conditions at conversational pace. After this period, progressively reintroduce more intense sessions.
The Sauna as an Acclimatization Tool
If weather conditions aren't warm enough yet where you train, or you want to accelerate adaptation, sauna is a validated tool. Spending 15 to 20 minutes in a sauna after a run triggers the same physiological adaptations as running in high heat. Studies show performance gains of around 2% on 5K events with a 3-week post-run sauna protocol.
Signs Acclimatization Is Working
After 10 to 14 days, you should notice: lower heart rate at the same pace in the same heat conditions, sweating starting earlier in your effort (sign the cooling system is more efficient), and faster recovery after heat efforts. If you don't see these after 2 weeks, make sure your runs are long enough (at least 40 minutes) and frequent enough (at least 5 days out of 7).