Continually (5,3,3)
(Other definitions for night and day that I've seen before include "twenty-four hours in length?" , "24 - 7" , "Cole Porter music" , "Play" , "All the time (Sinatra song)" .)
'continually' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for always that I've seen before include "On all occasions" , "Perpetually" , "Constantly" , "Continually" , "Irving Berlin's enduring song" .)