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! :)