Since i started LD'ing, one question have been standing unanswered.

When you want do wake up in the middle of the night/early morning to do a LD technique, you always set your alarm to go off in a multiply of 90 min, after you went to sleep, to hit REM.

I dont understand how this can be done unless you can go to sleep immediately. Normally it takes me at least 30 min to fall asleep, and this means that i will get 30 min to late for the REM.

Is there a logical explaination for this? Or can anyone but me go to sleep immediately?