One night, when I was still in Jr High I had a really bad fever. I had hot and cold flashes all night and I couldn't sleep (which I really needed because my parents wouldn't let me stay home unless I was dying).
It was summer at the time and the heat way more intense than the cold flashes, so I worked up the courage and grabbed the rotating fan in the corner of my room and plugged it in full blast next to my bed.
I think I had some fever induced paranoia because I kept thinking I saw someone watching me, but every time I opened my eyes I didn't see anyone (my parents were fast asleep at that time, believe me I checked). I blue light from the fan lit my corner of the room with a dim light that comforted me at the time so I tried once again to get some sleep.
I closed my eyes and listened to the fan, the sound got increasingly louder. The blue light that shown through my lids turned red and I felt my body get hotter, somehow the lower part of my body began to pulsate or throb with the sound of the fan (my first sexual experience and I was getting freaked out, go figure). I felt like I couldn't breathe, I tried to move or even speak, but I couldn't (this was one of my first experiences of sleep paralysis).
Then I started hearing voices, they were deep and scary, I couldn't hear what they were saying because it was jumbled and also because they didn't really seem like words. The got stronger as the voice did until it felt like it he/it was right next to my bed. The throbbing spread to my upper body, but were concentrated on certain areas. They slowly got stronger as the voice got closer. I felt the air getting thick and a scary feel in the air, like something, I don't know just plain creepy.
I was totally freaking out by then, somehow moved my head and everything stopped. I was afraid to fall back asleep that night, every time I felt myself drifting to sleep I would feel that presence or the throbbing and the fear again. I was about 3: 30 when it started happening that night, I was 13 and I was finally able to fall asleep around 5 as the sun began to rise. I don't know if it was the fever or my imagination or what, but it was frightening.
I swear I'm not making this up and I'm pretty sure I'm not crazy.
It's very unpleasant and usually a sign that you are very, very ill.