Pay attention and lose some sleep (3,2)
I believe the answer is:
sit up
'pay attention' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sit up that I've seen before include "form of exercise" , "postpone retirement" , "may aid fitness" , "Go to bed late" , "Abdominal exercise" .)