I am pretty sure I have undiagnosed ADHD(its pointless getting a diagnose in my country as an adult).

My resting heart rate is pretty high, but if I do any intelectual work wahtsoever my heart rate is around 95bpm.

RItalin analogues(in therapeutic doses) bring my heart rate to 60bpm when doing intelectual work while sitting on a desk -- during my deepest sleep my heart rate is 67bpm.

My own bro-science explanation is that I compensate my ADHD with anxiety -- when I'm anxious, I can focus just fine; some stimulants allow me to focus without the anxiety and that's why I feel deep calm and reduced heart rate while using them.