Is My Heart Rate Variation Normal?

I didn't really know where to post this. Perhaps someone here can help. This is a kind of odd phenomenon I've noticed recently. Before February of this year, my resting heart rate has always been consistently in the 60's. (I've been walking, biking, running, etc. even since I was very young). I'm 15 now, an have noticed an increase in my heart rate that seems out of place. I've still been exercising the same way and eating right. Here's the phenomenon:

In the morning, after I wake up, I can sit down and take my heart rate and it will always be 62-65 consistently. Then for the rest of the day, until the night, my heart gets to 84-100 bpm (which is totally out of place for my body). At night there is a huge dip in heart rate where it can go from 84-100 bpm to 48-60 bpm. I've talked to fellow runners/track people who told me that it varies only by 5-6 bpm from morning to afternoon. (Not sure how true that is, but they said for them it is a 5-6 bpm variation). Is this hormonal? What causes this? I'm 15 so perhaps my body is just going through massive changes and requires the heart to speed up.

I guess what I'm wondering here is if a variation of 20-50bpm from the morning to afternoon normal?

Thanks.

Big thing I hear wrong here is that you are getting no exercise. Your heart rate is NEVER above 100 bpm?

I expected to hear about PSVT (where the heart rate abruptly shoots up to 200) and the like!

Dora