Alright how does this sound:

Mana & Spell System: Everyone's Mana starts at 100, each spell will have a set amount of mana consumed per use. Every use of a spell levels it up by 1 level (or one experience point, your choice) Every ten levels/experience/uses will increase your mana capacity by 100. Note, classes don't EXIST in the roleplay, but if you do indicate you are a mage, you start with 300 mana instead of 100!

When you lose 1/4 of your Mana you will start to get slightly sleepy.

When you lose HALF of your Mana you will start to get tired

When you lose 3/4 of your Mana you will be exhausted.

When you lose ALL your Mana you will be fatigued mentally and physically, vulnerable to attacks.