Why is magic so exhausting?

I just rolled my second Wizard character, and I'm going around blithely killing enemies with my Magic Missiles. However, I'm seeing a message I never saw with my first character who used this spell.

Do you want to exhaust yourself casting this spell? [y/N]

If I choose yes, I receive another message.

Part of your life energy is drained while casting the spell!

My HP decreases in order to cast the spell. What is causing this? Is my Concentration too low to cast spells in quick succession like on my last wizard?

Solution 1:

If you don't have enough power points to cast the spell your health is drained instead. Spend a few turns walking around or waiting until your power points regenerate.