During exercise, is there something called a 'third wind'?

I work out with my heart rate around 85% of my maximum heart rate. That usually means Level 8 on my exercise bike. I hover around 85% of my MHR, but something happens between minute 50 to 60. My heart rates starts to decline and I have to move up to Level 9 to keep my heart rate up.

Is this a known phenomenon? Is there something physiological happening in my body?

get real on the third wind. there's really no such thing as a second wind unless you believe in made up stories

If this has been your workout routine for awhile then you are probably just getting conditoned to your max heart rate. If someone is athletically fit than they can exceed their MHR. I mean, think about it your heart is a muscle, as with any muscle once it is in shape it can take more exercise to keep it that way.

do you maintain the same cadence (pedaling rpm)? Because if you slow down, the resistance will have to increase to compensate for the less work done. so it could be you slowed down pedaling a bit

by the last fifety minuetes you are probably tired but don't want to stop your workout so you go to a harder level. this will make your heart ratego p again because more adrenline may be released like a sprinter finishing a race.

if you have wind you need to have less fibre in your diet.