The amount of HP healed doesn't lower if it's daytime or sunny. The regular HP recovery is 50%, and I think the nighttime recovery is 75%...
It's true that the amount goes up at night, but you may want to double-check the new percentage, because I'm not 100% sure about that one.
EDIT: Just realized that this was only the situation in Gen II, and possibly III. I've REALLY gotta get with these new times... -_-'