i always have a lot on my mind at nighttime.
do you sleep with a small light or a television on? i always try and pay attention...whenever i sleep in the dark i have dreams/nightmares about clowns and vampires. when i have a candle lit or my TV on and silent i don't.
it may have something to do with that? if you're hearing voices I would try adn observe a little more before you get completely paranoid about it or you'll become very stressed