Heart rate during exercise varies

[Dulce9]

I've noticed that in the morning, my heart rate often goes higher during exercise than it does doing the same exercise in the evening.  It can vary from day to day, too.  Sometimes I feel fatigued but push myself to exercise anyways, and I'm surprised to see that my heart rate is significantly lower than usual, by 25 to 30 beats.  From doing a little reading I see that this is pretty common, but I'm curious about other peoples' experiences.  What variables affect your heart rate?  What about a bad night's sleep, for example?

[Heybales]

I can have a 10-20 bpm increase from heat.

I can have a 5-10 from dehydrated.

If already warmed up I can have a lower reading at the start by 15-20.

Stressed from prior day and lack of sleep are big thing - so many variables I don't even attempt to map them out.

I don't get calorie burn from HR readings anyway, so that reason for accuracy isn't there - but I do like to have recovery HR runs the day after lifting or a hard bike ride, or if tomorrow is intended to be a hard workout, then HR matters.

[shipo · Accepted solution]

My heart rate varies based upon so many things it is hard to point to any one or two things.  The following is by no means an exhaustive list of things I've noticed:

• Too much sleep or not enough sleep.
• Too much exercise, or not enough exercise over the previous n-days or n-weeks.
• How much food, or lack thereof I've eaten over the last few days.
• Difficulty of workout (a flat 10-mile run will almost always yield a lower average heart rate than one with 1,000' of elevation change).
• Work/life stress.
• ...
• And the list goes on.