So, i am in the middle of a WBTB right now. I spontaneously awoke after about 4 hours this time. My question is the following: are REM cycles constant to the minute? I mean, if i woke up after 4hours, would my cycle probably be of eirher one or two hours? Or is it usual that during the night, a person's cycle deeply vary, like one ending after half an hour, other after one and a half hour and other after one hour?