Inhale, heart speeds up; deep exhale, heart slows down. Why?

I’ve noticed that when I take a deep breath in, my heart rate will get faster. When I slowly exhale, my heart rate slows down drastically, and stays low for a few moments afterward, then returns to normal.

Is this common? What’s going on in there that causes this to happen?

Heart-Rate Variability (HRV) is completely normal, though in some people this effect is more pronounced than in others. Here’s a page with some technical info about it: