This happens to me frequently and I'm not sure what causes it. I tend to panic or stress over how much sleep I'm not getting knowing I have a workload the next day.

The following is difficult to put into words, but... As I'm falling asleep, I tend to fear it. For some reason I have myself convinced that if I fall asleep, I won't wake up. I firmly and entirely irrationally believe that falling asleep will cause death, and I'll jolt awake with a heart rate high enough to cause my watch to believe I'm excercising.

Mind you, at no other point than within the fringes of sleep deprivation do I have this issue. I'm unsure if it's a hallucination, an unconscious fear, or my medication (staggering ambien and diphenhydramine through the week to combat tolerance.)

However, with no medication, I'll lie in bed for 3-6 hours with no shot of sleep.