Wiktionary
happy as a lark
a. (context simile colloquial English) Very happy (sometimes with the extra connotations of being carefree or unaware of grimmer realities).
Usage examples of "happy as a lark".
I'm well aware that if you feed a man enough of that Happiness drug, he'll be happy as a lark even if you cut off his arms and legs and then start on his head.
How man discovered what makes hydrogen go boom and was happy as a lark while it went boom and boom and boom?
The Professor looked happy as a lark and not a little proud after having revealed himself to be the original discoverer of the thing.