Guanfacine | ADHD Information

My six-year-old son just started taking Guanfacine (working up to 1 mg at 7 PM and 1 mg at 7 AM).  His teacher has seen improvement in his ability to sit still and get his work done.  Unfortunately, he has been having a hard time sleeping at night and wakes up at least once during the night saying he is "scared."  This started happening only after starting the medication.  Anyone else have this issue?  Our 10 year old daughter takes Melatonin to help her get to sleep and I'm wondering if it would be safe to give our son both Guanfacine and Melatonin at night?  Thanks for any input!