Originally posted by Leo Volont
Actually, REM cycles tighten up as the night progresses. Only the first REM cycle is out by 90 minutes. when setting a progressive timer, it is perhaps more realistic to set for the first beep at 90 minutes, then the next for 80, and then 70, then 60.
Umm... I always thought that it goes like this: Every 90 minutes you start a new REM-period. The first period starts 90 minutes after you fall asleep, but it's pretty short and still quite deep sleep. When the night goes further on, REM-periods will become longer and longer and they won't be so deep anymore. That's why you have a greater chance to remember those morning dreams and become lucid during them. So it would be something like 90min deep sleep, 10 min REM, 80 min deep slee, 20 min REM, 70 min deep sleep, 30 min REM, 60 min deep sleep etc.

Well, maybe someone, who really knows about this stuff, can come and tell how it really is.