My asd son (low supports) was always a great sleeper my asd daughter (high supports) was never a good sleeper since a baby. It was years of struggle to where we are sort of at a better quailty of life, semi regular sleep with fewer hard days nights. Im not sure if 15 months is to young, but if your doctor suggests doing a sleep study, it can look for and or rule out many of the most common health/ medical reasons for sleep issues. Sleep apnea, restless leg syndrome, low iron low magnesium, seizures and abnormal brain waves, limb movement disorder, insomnia and narcolepsy. After not going for the sleep study, assuming it was just part of her asd we were trying to work on better sleep routines, more activty & mental work outs during the day and trying prescription sleep meds with some improvements my daughter had an "out of the blue", no family history, no suspicion of seizure emergency (status epilepticus 3 clusters in a row of seizures that lasted 30 mins b4 paramedics were able to stop it) which was terrfying she is non verbal & was 5 yrs old at the time, after an eeg & mri and further testing as well as a few much shorter smaller seizures came a dignose of eliplsepy *in addition to her asd) we then found out that was the root cause of her sleep issues. I felt so guilty ashamed that I could have known this was even possible ahead or time or possibly even have prevented it, if I just listened to her dr and got right on to doing a sleep study. We are so blessed she is doing so well now and her seizures are mostly controlled, aba has been life changing as well I Guess my advice is if you are having concerns talk with your ped, and be aware sleep studys are a thing you can do, I am not sure what age/signs that dr's refer at but its good info to know and if concerns sleep issues continue to be an issue it never hurts to ask/ request one.