That's the punishment for his excessive blood drinking. Mettaton feels exhausted, but he keeps talking to try and play it off, even if he can't really move. He'll let Liquid deal with it. It's only fair.

"I have never felt such an intense need to take a nap...goodness, aren't humans fragile? And here I thought you were all so much more sturdy."

What does that say about monsters? Haha...ha...