Now this is one that I take for granted. I never even thought of it as a phrase.
Well, it seems that
before
box springs were in use, old bed frames used rope pulled tightly
between the frame rails to support a
mattress. If the rope became
loose, the
mattress would sag making for uncomfortable sleeping. Tightening the ropes
would help one get a
good night sleep.
Now you can rest easily with that bit of information. Sleep tight! :)