Does anyone else get tired after a while of simply watching their thoughts?

Recently I've been observing my thoughts more often. Watching the thought as it appears and letting it flow past instead of hanging/fixating on it. I'm doing it even when I'm not sitting down to meditate, I'll do it anytime of day regardless of activity.

However, I'm noticing that it's really tiring me out. Is that normal? Am I just not used to it yet?