According to the sources provided, Lipitor, a medication used to lower cholesterol levels, may cause insomnia or disrupt sleep patterns, but there is no specific information on how long it may affect REM sleep.

A study published in the Journal of Clinical Psychopharmacology [3] found that atorvastatin, the active ingredient in Lipitor, was associated with sleep disturbances, including insomnia, but the study did not specify how long these effects may last.

AARP [2] lists Lipitor as one of the medications that can cause insomnia, but again, there is no information on how long the effects may last.

Drug Patent Watch [1] provides information on the patent status of Lipitor but does not provide any information on its effects on sleep patterns.

In summary, while Lipitor may cause insomnia or disrupt sleep patterns, there is no specific information on how long these effects may last or how they specifically affect REM sleep. It is recommended to speak with a healthcare provider if experiencing sleep disturbances while taking Lipitor or any other medication.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.aarp.org/health/drugs-supplements/info-04-2013/medications-that-can-cause-insomnia.html
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2699168/