The good news is that it’s not just your imagination: “The human body is an incredibly inefficient organ, in that when we exercise only about 25% of energy is used for movement – the other 75% is used for heat production,” says Professor Mathew Wilson, Head of Service for Sports Medicine for HCA UK.

This means that a 5K run in the blazing sun really is harder than in cool, breezy weather because of the ‘thermo regulatory stress’, which makes your cardiovascular system work extra hard. “The body is clever and sends more blood to circulate through the skin in order to cool itself, however this means less blood for your muscles and results in a higher heart rate,” explains Susie Wright, therapies manager at the Princess Grace Hospital.

“When there is high humidity too, the stress on the body increases as it means sweat does not evaporate as easily from your skin, resulting in your body temperature rising.”

Just like your brain can struggle to concentrate on more than one thing at a time, the body can be too preoccupied with keeping cool to up the pace. Meaning if you do push harder, your body will be working in overtime.

UK heatwave: how to exercise safely in hot weather, according to experts