How can I help Mother with disturbed sleep cycles with dementia?

My mother goes through increasingly disturbing sleep cycles for the past few years, worsening lately now that her dementia has significantly progressed. She will sleep fine for a few nights, then not sleep at all (for the most part) for the next 3 nights. This same cycle repeats week after week, and is now getting to the point where she is SO agitated that she stays in her room all day long after a sleepless night. The nurse told me that even though mother complains of not sleeping at all, when she's checked on during the night, she IS asleep sometimes, at least for a short while. In her mind, she is not sleeping 'at all' and tells me she didn't sleep for even 5 minutes all night long. On Wednesday night, it was 'the worst it's ever been', then on Thursday night, it was 'even worse than Wednesday night, NEVER this bad.' And on and on.

Has anyone had a parent with this issue, and if so, were sleeping pills prescribed? If so, which ones? Mom has always had sleep issues, and was taking Ambien for a few years; wound up breaking her foot while using the bathroom in the middle of the night (this was in her pre-dementia days), so the medication had to be discontinued. I know the doctors are leery to prescribe sedatives or sleep aids with dementia.

Any advice? Melatonin doesn't work, switching the Gabapentin from day to night dose does not help, nothing seems to break this 'cycle' she's on.