Your heart rate at a given effort is not fixed. Heat, dehydration, a short night, stress, caffeine, alcohol and a brewing illness can all push it higher, so the same easy jog can read noticeably faster on a hot, tired day than on a cool, rested one. That is normal physiology rather than lost fitness, and it usually settles once the cause passes.
It matters if you train by heart rate, because a number that has drifted for an ordinary reason can make an easy session feel like a failure, or tempt you to push harder than your body is ready for that day.
What can make your heart rate higher than usual?
| Cause | What is happening | What to do that day |
|---|---|---|
| Heat and humidity | Heart rate drifts upward as more blood is sent to the skin to shed heat | Go by effort and the talk test, slow down, choose a cooler time of day |
| Dehydration | The rise in heart rate during exercise grows with the amount of fluid lost | Drink through longer sessions, especially in warm weather |
| Short or poor sleep | Lack of sleep is a recognised trigger of a racing heart | Keep the session easy, or swap it for a walk |
| Stress and anxiety | The stress response raises heart rate before you have moved | Expect a higher start and judge by how the effort feels |
| Caffeine, alcohol, nicotine | All are listed by the NHS as common triggers of palpitations | Note them if your numbers look unusual |
| Infection or illness | A fast pulse can be a sign of infection or dehydration | Rest rather than train through it |
| Long sessions | Heart rate creeps up over time even at a steady pace, known as cardiovascular drift | Expect a later rise and do not chase the early number |
Why does heat raise your heart rate?
In warm conditions your body sends more blood to the skin to lose heat, which leaves less returning to the heart with each beat. The heart compensates by beating faster. Researchers call the resulting steady climb cardiovascular drift, a continuous rise in heart rate over the course of a session.
A 2023 study of 51 young adults exposed to progressively hotter or more humid conditions found that heart rate began to drift upward before core body temperature started to climb. In their protocols, cardiovascular strain rose roughly 20 minutes before the heat became more than the body could offload, even during very light activity. In other words, your heart rate is often the first thing to tell you the heat is winning.
How much does dehydration matter?
Quite a lot, and in proportion. In a classic 1992 study, eight trained cyclists rode for two hours in warm conditions (33°C) four times, drinking different amounts each time. They ended up between about 1% and 4% lighter from fluid loss. The more dehydrated they became, the more their heart rate and core temperature rose: the authors found both increases were graded in proportion to the fluid lost.
The small sample is typical of lab physiology, and these were trained athletes in the heat, so the exact figures will not transfer to everyone. The direction is consistent, though: the less you replace what you sweat out on a long, warm session, the higher your heart rate climbs for the same work.
Can poor sleep or stress change your heart rate?
Yes. The NHS lists lack of sleep, stress and anxiety among the common triggers of heart palpitations, alongside strenuous exercise, alcohol, caffeine, nicotine and some medicines. Our guide to how the stress response works explains why your heart speeds up when you are under pressure, even sitting still.
These triggers also stack. A short night followed by two coffees and a tense morning can put your heart rate noticeably higher before a workout has even started, which is worth remembering before you decide the session has gone badly.
Should you still train by heart rate on those days?
Use it alongside how the effort feels, not instead of it. The US Centers for Disease Control and Prevention describe a simple talk test: at moderate intensity you can talk but not sing, and at vigorous intensity you cannot say more than a few words without pausing for breath. The talk test adjusts itself to heat, tiredness and stress in a way a fixed number cannot.
A practical rule on a hot or tired day is to keep the effort, not the number. If your heart rate reads high but you can still talk comfortably, you are working at the intended level. If it reads high and you are struggling to speak, ease off.
Our target heart rate calculator gives you your zones, and pairs each one with its talk test, so you have both to hand.
When is a high heart rate a reason to see a doctor?
Day-to-day variation with an obvious cause is expected. It is worth getting checked if your resting heart rate stays high after the cause has passed, if your pulse feels irregular, or if palpitations keep coming back, happen more often or last longer than a few minutes. Our guide to what a normal resting heart rate is covers the ranges and when a reading outside them matters.
Stop exercising and get urgent help if your heart is racing and does not settle, particularly with chest pain, shortness of breath, or feeling faint. In the UK the NHS advises calling 999 in that situation, and not driving yourself to hospital.
