sleep tight!

Every time Alexa tells me to "sleep tight" I smile, because I know where the phrase came from.

Rope beds like this one were once very popular.  You'd have to tighten the ropes periodically to keep them from sagging, to keep the mattress from hitting the floor.  

The mattress would have been filled with hay.  You used a tool that looks like a giant rolling pin to smooth out the mattress. And if a piece of hay poked out of the ticking, you'd hit the piece until you could force it back inside.  Hence the phrase "hit the hay".