I agree LS you do need to trust your children, but they don't need the phone/computer in their rooms at night. That's asking for problems. The reality is if they have the phone/computer in their rooms it will be used.
I disagree. When DS1 says he's going to bed,he does just that. He switches his phone to airplane mode (so he can still use it as an alarm but it stops unwanted calls/sms throughout the night) and turns off his laptop. He's been like this since he first had a phone at 14.

Do I check that he's done this? No I don't. I don't feel the need to. He tells me that this is what he does and I believe him.

When I told my stepdaughter it was time to get some sleep and to shutdown her computer she did just that. It was her choice to give us her phone at night because her brother was calling and friends were sending through text messages.

I use the same motto on my kids that my mother used for me: if you can't do it in front of me then it must be wrong. In other words, if you have to hide and don't want me to find out, then maybe you shouldn't be doing it.