Morning sun is based off the weather. In rain, it restores health 1/4. In sun, it restores 3/4. So what about in desolate land and primordial sea? Would it fully restore you in desolate land, and do nothing in primordial sea?
Logically, yes. But his is Gamefreak so no logic here. Still 3/4 in harsh sun and 1/4 in heavy rain.
Hope this helps!