It will certainly be the unusual case if she has a sleep disorder that won't result in a stable sleep/wake cycle. I don't think you can do anything at this point to promote sleep onset. Be sure she is falling asleep on her own (vs. rocking her, holding her, etc). Also refrain, if relevant, from the use of bottles or pacifiers in her crib. Continue with your practice of allowing her to resume sleep on her own during the night. It is possible that the prematurity is a factor in what you are experiencing, but over time she will likely establish a pattern of sleeping through the night.
My twins are five and one half months of age, not twenty-two months.
The sleeping arrangements are as follows. For nightime sleeping, my husband sleeps in our bedroom with one baby and I sleep in the girls room with the other baby. Both rooms have cribs.
When my daugther typically has no trouble falling asleep, but after about thirty minutes she wakes. I have tried letting her cry after she wakes up, thirty minutes after her nap began. For one week, she would fall asleep, for a nap, I would put her in her bed, she would wake thirty minutes later, I would then let her cry. She would cry for an hour, sometimes longer, I would then go in and get her. I would assume letting her cry for an hour was long enough.
However, using the same strategy at night would work. She would cry for maybe 10-20 minutes, then go to bed. She would then awaken anywhere from 2-5 hours later.
She then typically wakes up every 2-3 hours for the rest of the night.
She is fed approximatley 75% breast milk and 25% formula.
What happens when your daughter wakes up at nap time, and what happens when she wakes up during the night? What is the napping and sleeping arrangements? Knowing this will help me offer you some guidance, though it may be that you really can't do much to alter this situation.