A girl is sliding her shirt up in her "sleep" every night and I can see her nipples. It's been four nights in a row. Is this on purpose?

It's difficult to determine someone's intentions without directly asking them. There could be various reasons for her behavior—she may be adjusting her shirt in her sleep without being aware of it, or it could be a conscious choice. If you're comfortable, the best approach would be to have an open and respectful conversation with her about it. Communication is key in understanding someone's motives or feelings.