i get them too, they are called Hypnagogic or Hypnopompic hallucinations. i dont think i had one for several weeks but the last one i had was a bug crawling on the cieling. the youngest i remember having them was around 6 or 7. but my mom told me i was screaming about a man with a gun when i was around 4 i think. i read up about them some and found out they are usualy associated with a sleep disorder and have a greater chance of occuring when your tired.