I've noticed the same thing when I ride in my basement. If I come up off of the hoods, grab a water bottle and take a drink my heart rate bumps up ~5bpm. It'll

I've noticed the same thing when I ride in my basement. If I come up off of the hoods, grab a water bottle and take a drink my heart rate bumps up ~5bpm. It'll climb a bit more if I eat something like a granola bar or a gel. What's interesting is that unlike steady state to a hard sprint, the heart rate rise is almost immediate. There's some delay - 20 to 45 seconds - if I change it up and do a sprint interval before my heart rate reflects that effort.