When do the sleep stages actually begin?

[Repo!]

Like the title asks: when do the sleep stages actually begin? I'm wondering because I'm trying to time it to wake up around my REM stages but can't seem to time it right. I think theres a problem...I usually meditate for a good 20 minutes before bed. I do this to clear my thoughts and to reflect on the day. Then, after this, I go to sleep.However, it takes me anywhere between 20 minutes to 45 minutes to fall asleep. So back to my question...when do the stages actually start? Because although I am not completely unconscious and asleep, I am in a extremely relaxed state. I'm trying to aim to wake up around a REM period, so can I have some advice as to when I should "set the timer" to wake up? Thanks! -repo!

[Supernova]

Unless I am mistaken, they begin as soon as you actually fall asleep fully.