Each upvote you cast reduces your VP (Vote Power), such that you have ~10 upvotes per day at 'full power', and thereafter the value of your vote declines. Because of this I usually (always) vote at 25% power, so that I can cast ~40 votes per day because I read a lot and upvote the great content on Hive.
Hope that helps.
Edited for corrections.
Each vote cast reduces RC, not HP. Each upvote cast increases HP.
Yeah, and each upvote reduces the value of the next one, change starts to be significant after these 10votes daily are reached
Voting Mana (VM) is responsible for that effect.
If your VM is 100%, your vote is worth maximum potential. It would then take roughly two hours to fully charge VM back up to 100% after casting that vote. I try to keep mine above 90% at all times.