hysterical at the thought of not having their phones in their hands all the time
Extremely common. Phones are a massive part of their lives now. They would feel out of the loop without them. It's such a personal thing - so much more than a functional device: diary, photo album, address book, magazine.....
It's cool to text/FB/tweet etc very late at night and to text rather than phone even very close friends. Their messages are sent in the expectation that adults won't see them. Even gentle, nice girls use language that we wouldn't. They're addictive IMO. Checking for likes on instagram, FB, messages on What's App deliver a little hit of dopamine apparently.
The etiquette among most teenagers is that it's considered rude not to reply to texts, even late at night and it's fine to text someone until they don't answer as they've fallen asleep.
No advice from me - I battled with my DD over her phone until she was 15, then pretty much gave up; one of my parenting fails. On the other hand, now she's 17.5 and voluntarily moderating her phone use and sleep pattern reasonably sensibly and I think I have quite a good relationship with her.