You can't feed Mogwai after midnight because otherwise they turn into Gremlins.
But what time is safe again?
There's no real explanation given, and in fact these questions are raised in the film series itself.
There's also no rational explanation given for the transition of a mammal into a reptile. It just happens.
The only possible explanation is magic.
Following the common themes of magic, I would assume it's a problem between the witching hour (midnight) and dawn - dawn being the typical start of a new day, with new beginnings, and generally presumed to have mystical significance.
Having seen the movies as an older person, (lol) my thought on it is that it was meant for kids. Therefore it is a warning against midnight snacks- no food after midnight. Just like children, they would be going to sleep and then eating in the morning. So, I think that's what makes them "safe"- a good nights sleep before eating.