What on earth does this mean (Sleep Problems)?

I randomly woke up this morning at 4 a.m. My eyes were tired but my body was wide awake. Each time i tried to sleep, i experienced a sickening sensation where every sense heightened (especially hearing) and the feeling it created was equivalent to that of fear, or that feeling you get when you go down a really high roller coaster hill. It took several tries to get to sleep because this would happen every time i almost got to sleep. I slept for about three hours after that and feel i have not rested at all. What on earth does this mean?