You're tired, but not ready to quit yet. You decide next to practice the Time Freeze spell.
It seems relatively simple enough. Just shout the command words, and everything else is fine.
"Time Freeze!" You shout at the top of your lungs.
Suddenly the entire world seems to go into gray scale. Time is frozen. Though, you're unsure of why that made the world go into gray scale.
You don't ponder on it long though, because you suddenly feel extremely fatigued, and it takes most of your effort just to stay awake.
Your concentration slips as you try to force yourself to stay awake, and time unfreezes.
Well, the spell works, you just weren't able to maintain it for very long. Of course, you've done a lot of casting today on top of your practice, so it's not really much of a wonder why you're so tired.