have they tried melatonin?
22 mos is still quite little though - not desperately unusual to be wakeful during the night. have they got any sleep advice from the gp/ hv already? or has it all been exhausted?
agree that i'm not sure i would be treating it as 'part' of the condition unless the paed agreed - what does her paed say? the paed would be the one to discuss melatonin with, but she might be too little to consider it yet.
given that the coloboma/ hyperactivity link in mice is reversed with stimulants, (no idea about kids) presumably it means that at an older age (usually 6 or so) other meds can be trialled - standard adhd meds like ritalin, which might be beneficial.
but i would be suggesting they discussed sleep issues with the paed, gp or hv (does she even have a paed/ eye consultant for coloboma? i'm guessing hers is rather more severe, not the m mcc variety)
sometimes i have a tendency to attribute issues to dd2's disability when really they are just run of the mill childhood things (she went through the usual night terros thing, which at the time we thought might be muscle spasms etc, but as she was non-verbal we couldn't tell. it certainly sounded like she was in pain for hours!) so now i always try and go for the 'ordinary kid' option first. 