system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my music sound slower when I exercise?

This is tough to describe. When I'm listening to music on my phone, it has a usual rhythm. But when I listen while I exercise, the music sounds just a touch slower. 

But, when I think about how slow it is, the feeling goes away, and the music sounds regular again.

I imagine this has to do with focus, but I'm sure someone can explain it in depth.
gpt
Your body's internal clock is set based on your heart rate, which usually is the exact beat of the BeeGees' "Staying Alive". When you exercise, your heart rate increases to the beat of other, better songs.