Why does your heart rate drift during a long run?
You're holding the same pace and the effort feels steady. But an hour in, your watch shows your heart rate has crept up by ten beats or more.
Before you decide you're getting unfit, know that this has a name: cardiac drift (or cardiovascular drift). It happens to nearly everyone on long efforts.
Your heart is working harder to deliver the same output
As you sweat, the fluid part of your blood shrinks. With less blood returning to the heart, each beat pushes out a little less, so your heart beats faster to keep your muscles supplied with the same amount of oxygen.
Heat adds to it. Your body sends more blood to your skin to shed warmth, which leaves less available for your muscles and pushes your heart rate higher again.
Part of the drift is fixable
Studies comparing runners and cyclists who drink with those who don't have found that the drift is bigger when fluid isn't replaced and smaller when it is. So some of that climbing number is your hydration, not your fitness.
A little drift is normal
A modest rise over a long, steady effort is expected. Coaches often treat a change of around 5% or less between the first and second half of a steady run as a rough sign of a solid aerobic base. It's a rule of thumb rather than a hard line, but it shows that some drift is just physiology.
A FEW THINGS WORTH TRYING
Compare your own runs. Look back at runs on the same route in similar weather and note how much you drank on each. If the drift is smaller when you took on fluid early, you've found your lever.
Take on fluid and sodium early. Start sipping in the first half hour rather than waiting until your heart rate is already climbing.
Go by feel on hot days. If you're chasing a heart rate zone in the heat, you'll end up slowing right down. Use effort as your guide instead.
Keep cool where you can. Shade, a cap and water tipped over your head all reduce how hard your heart has to work.
Check your app. Some show "decoupling" or "aerobic drift" after each run, which makes it easy to spot trends.
See a doctor if your heart rate is unusually high at an easy effort day after day, or if you get chest pain, palpitations, dizziness or breathlessness that seems out of proportion to the effort. Those aren't hydration problems.
Drift isn't a sign your body is falling apart. It's your body doing the maths on a long, hot effort, and you can often nudge the answer in your favour.