Going to sleep tonight.

Ghosts.
The question is not whether they exist or not, but rather if your sanity is ready to make them part of your reality.

And to awnser >>1's question..

Sanity makes you hallucinate, once you see it your fear grows stronger and so does your sanity, after you surpass sanity, fear grows so strong your body will wreck itself and you will die of fear eventualy.