I’ve always found it strange that it’s so easy to stay up until the next day. Like staying up until midnight isn’t that crazy, so it’s quite easy to find yourself in “haha it’s now the next day!” territory.
But it’s not really true. The day really ends when I go to bed, and starts when I wake up. The actual time doesn’t matter.
Perhaps midnight is 12am because it’s about halfway between sunset and sunrise. But humans (at least since the invention of artificial light) stay awake past sunset but generally balk at the idea of waking up before sunrise.
So midnight needs to be shifted more towards the true center of the night, rather than the sunrise/sunset center of the night.
On a given night, if you’re still awake at 3am, it’s probably an anomaly (unless your sleep schedule is just like that...). So it’s a safe time for the day to switch over, and when you wake up it’ll be the next day. Clean and simple. The day should end at 3am.