I was a poor sleeper my whole life and suffered from sleepwalking and night terrors all the time as a kid. I have a bunch of terrifying experiences from that time period, but the one that sticks out the most was one night when I was around 17.
My bedroom was pitch black, being a long basement room with only one tiny window that got no streetlight at all. In the middle of the night one night, I got a chill that ran up my spine. Something else was in the room with me. I sat bolt upright in bed and scanned the darkness. There, standing right under the window was a girl. She was probably 15 or 16 years old, with curly blonde hair and wearing a sleeveless red plaid shirt. I was sure this was one of my sleepwalking episodes, so I did what I normally do in those situations, shook my head back and forth, threw my covers off and rubbed my eyes....shit, she's still there.
We locked eyes for what seemed like a minute, neither of us moving. Then, the girl took a couple steps backwards, which took her out of the minimal beam of light coming through the window and into the inky blackness. Terrified, I leaped out of bed and turned the light on in my bathroom right next to the bed. Nothing...she was gone.
I'm sure I was just having another night terror or sleepwalking episode, but I never shook that night. 18 or 19 years later and I can still remember her face.