There's so much you shouldn't read when you can't sleep, really. You can start with a simple pilar cyst on Wikipedia and keep clicking related links until next thing you know you're looking at a section of a fossilized fetus that was in a woman's body for 40 years... nightmares forever...
Incidentally, I first found out about teratomas years ago in a story by Margaret Atwood called "Hairball", which is a fantastically creepy revenge fantasy. It's in the collection "Wilderness Stories" which has a bunch of good stuff, if you're interested.
Gross. There was a CFL (Canadian football) player who suspected he had a brain tumour of some kind but his doctor brushed him off until ONE DAY HE SNEEZED OUT A TOOTH. FROM THE TUMOUR. A TOOTH FROM HIS TUMOUR CAME OUT HIS NOSE.
FROM A TUMOUR
IN HIS HEAD
IT CAME OUT HIS NOSE