I take Atarax. About a third of a 5 mg tablet, on an empty stomach (tres important) with water (not milk). Makes me groggy, but rested, till about 11 a.m. Haven't needed it for several months though.

I don' know about REM and all that, but I know the other portion of what you wrote. After it happened to me a couple of times, I take it without supper and with plain water. That way, all 5/3 grams get absorbed immediately and knocks me out. If it goes into a full stomach, it takes hours for the thing to enter the bloodstream and the concentration in the blood would be too low to knock me out.