DD is an early bird. Sometimes she'll play happily in her room when she wakes for 30-45 mins before coming to get us. Other times, she'll come racing through as soon as the suns up, ready to start the day!
DH has suggested getting a Gro Clock which I'm up for in principle, but I think that at 20 months she's a big young to really get it. Her understanding is generally good (e.g she can follow two-part instructions and can be reasoned with in simple ways (if you do x then I'll do y).
(I should add, her room is perfectly safe - before anyone says 20 months is too young to be left alone - and the door is ajar so she's not shut in there! The only place she can access when she leaves her room is our room and there's no danger on the way)
I think that might be a bit young. DD was about 2.5. It was miraculous though, she went from getting up at 5.30am every day to staying in bed til 7am. She still woke up half the time but just stayed in bed staring at the clock until it told her she could get up