Not up? But you may be up if you are here! (10)
I believe the answer is:
downstairs
I believe this is a double definition.
'not up?' is the first definition.
(not upstairs)
'you may be up if you are here' is the second definition.
(you go downstairs when you wake up)
'but' acts as a link.
(Other definitions for downstairs that I've seen before include "On a lower floor" , "towards lower levels" , "in the cellar?" , "On ground floor (or in basement)" .)