Night Sky

It's not just the day and night cycle, temperature and phases of the moon Princess Celestia and Princess Luna are responsible for in Equestria, although you knew it, but you did not want to make the posts too long to make it easier for people to read, what is shown in MLP official, is Princess Luna takes care of psychological problems for Ponies in their dreams as well. She did this for Cutie Mark Crusaders, offered them support to deal with bullying and insecurities about not getting their cutie marks.

I just wished the episodes had been more thorough with the explanation, realistically Princess Luna should have been a qualified therapist to make this less controversial, shall we say? like if we were actual citizens in that fantasy realm Equestria, it's logical to deduce her to have had some formal education on psychiatry and psychology if she goes out of her way to use dreams to help Ponies successfully deal with their fears. If she hadn't, that would not look good for her.