Reason you feel exhausted in a heatwave as ‘hottest day ever’ on its way 

The UK has experienced an unprecedented summer when it comes to the hot weather, experiencing three major heatwaves. This year marks the first time in British history that temperatures have breached 35C in three separate months (May, June, and July).

https://www.mirror.co.uk/news/health/reason-you-feel-exhausted-heatwave-37458366