I can't tell you how many times I hear parents complain about their kids being tired, sullen, withdrawn, etc, etc, etc. My first question always is, "Do you allow them to have their phones in their rooms at night?"
I kid you not. 99% of the time the response is "Yes, but that's because they use it as an alarm clock. They aren't on their phone at night." RIIIGGGGGGHHHHTTTT. Sure they aren't.
My first recommendation to the parent of a tired and crabby tween or teen is to buy them a super simple alarm clock that actually (gasp) plugs into the wall and take their phones at night and charge them in your room. Sure, you can charge them downstairs in the kitchen, but if they have a habit of being on their phones at night, I guarantee you they will eventually sneak downstairs or wherever the phone is to check it in the middle of the night. And when you hear them, get out of bed and flip the kitchen light on, they'll be all "Oh, I was just getting a drink of water!". Sure you were.
So. Invest the $20 for a regular 'ole alarm clock, take their phones at night and while they may be mad at first, each day that anger will lessen and they'll start getting more, better sleep.
And if you haven't given your child their first smart phone but are preparing to, prevent that habit from starting simply by gifting them an alarm clock along with their phone. Set the "no phones while sleeping" rule right from the start and it won't ever e an issue.
Remember, habits form regardless. It's up to you if your children form good ones or bad ones.