Fatigued: is your heart rate lower or higher.

What happens to your heart rate if you are getting tired ? Or what do you think you are when your heart rate is higher or lower when your assumed base line? I see different posts where people say oh i am tired my heart is higher than usual, or they say the exact opposite. Is it the same when your heart rate changes during the course of the same ride and over the course of several days. In my case it always goes down for both cases whether I get tired during the ride or if I accumulate fatigue over several days. The difference for me comparing fresh ride and being tired is about 10-15bpm.