I'm a young adult with SPD, and I've had issues with insomnia pretty much my whole life, as have my brother and cousin with SPD. I tend to have trouble sleeping because when I settle down I become really oversensitive to noise, touch of fabrics and even small vibrations of my bed. When I was 10ish I had a whole week where I couldn't sleep and I heard this agonizingly loud beeping and it turned out that the smoke detector in our basement three floors down was out of batteries. I have to distract myself and block out noise in order to get to sleep, because if I'm distracted I'm less likely to be bothered by small irritants. Listening to audiobooks with headphones in works best for me (I've probably listened to the harry potter audiobooks hundreds of times), but I will also listen to calming music and distract myself by telling myself stories. I've never tried white noise, but it works well for my cousin (who's 13)