Sleep Disorder

I have fraternal 22 month old twin girls, that were born seven weeks premature.  One of the twins is an excellent sleeper.  The other twin has no difficulty falling asleep, but she simply won't stay sleeping.  She will usually wakes thirty minutes after falling asleep, for naps.  She has longer periods of sleep at night, but her naps are limited to thirty minutes.  Sometimes she can be soothed back to sleep, but then will awaken thirty minutes later.  
She goes to bed at around 7:30 p.m., with the same nighttime routine,   wakes up between 12:00 and 1:00 a.m., goes back to sleep, then usually wakes up every two hours until around 8:00 a.m.
I have taken her to her pediatrician, nothing is physically wrong.  He gave  me some benadryl, which I gave her once and did not help her fall asleep.
She is so over tired from not napping enough in the afternoon she is very fussy and cranky.
I would like to know if there is anything I can do to help her stay asleep.
Also, her doctor mentioned that she might have a sleep disorder.
I would appreciate any suggestions.
Thanks.

[Kevin Kennedy, Ph.D.]

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.

[Angtwins]

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.

[Kevin Kennedy, Ph.D.]

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.