My DD is the same. Now she's 18 she uses it slightly less I think.
The difficulty in trying to help in this situation is the fact that it seems to be almost normal to be 'on' your smartphone about five hundred times a day for 'today's' teenagers, so your DD probably thinks you're old and out of touch with what it's like to be a teenager today and won't take any notice.
Apparently the anticipated, but not certain, 'reward' of seeing a like/comment/message delivers a hit of dopamine, which I don't need to tell you is addictive. And as you probably know, the more 'rewards' obtained like this the less sensitive your DD will become to other, healthier stimuli that would normally afford pleasure and feelings of well-being. So it's a real problem IMO. Not just because it wastes so much time.
I'm sorry I can't link to specific research. Your DD might think you're kicking up a fuss over nothing, given that she is doing very well at school but showing her info on addiction might help. (You know her better than we do
) (My DD admitted that she probably was addicted, but wasn't willing to address the issue.)
I think teenagers who have their smartphone-use restricted probably feel very 'controlled' - could you do a deal perhaps? You won't comment on something else/will offer more freedom in a different part of her life if she agrees to using her phone less? How about discussing her having a phone that isn't smart, so that at least she doesn't have the constant temptation of snapchat/instagram/fb etc on her phone?
FWIW, this is one area of parenting I feel I've done badly. No problems with drinking/smoking/unhealthy eating/etc (so far anyway) but I wonder if DD would have achieved more and have a better attitude to some things if she hadn't spent so much time on her phone.
Then again, perhaps she'd have done something else instead.
DH and I tried to get her to cut down her use for Lent as she usually does something for Lent, but she wouldn't even consider it.