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I am sorry to hear about the problems you are experiencing with your granddaughter. While it is somewhat normal for teens to have changes in their sleep and wake schedules over the summer, what you are describing is more than we might expect. I would assume that she will be returning to school soon, and as such, will need to be returning to a more normal schedule. Did she have this same pattern last summer, and if so was it easily corrected? One of the best ways to change these patterns is by gradually adjusting them, for instance she could begin to go to bed 30 minutes earlier each night to get to a more regular bedtime. This would be possible only if your granddaughter is interested in working on this issue. However, if she is not wanting to change this, then it could be more difficult of a problem. I would wonder if your granddaughter is avoiding family and friends by staying in her room all night and sleeping all day. Are there family problems that she is trying to avoid? Withdrawal and sleep changes can sometimes be a symptom of depression. If she is not interested in making changes to improve her sleep schedule, then it may be helpful for her to be evaluated by a psychologist to determine whether depression may be an issue. In addition, a physical evaluation including blood work to rule out any medical issues could be important. No matter what the cause, unfortunately nagging will not improve the situation. Letting your granddaughter know that you care about her and are willing to help; offering unconditional love would be helpful.
I hope this answer is helpful. Please let me know if I can clarify further.