I'm often unable to get off between the sheets (9)
I believe the answer is:
insomniac
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for insomniac that I've seen before include "Can't drop off" , "who won't get off?" , "One unable to sleep" , "Habitually sleepless person" , "One lying awake" .)