What is the meaning of the idiom 'don't let the bedbugs bite'?

Because bedbugs hurt when they bite! You say "don't let the bedbugs bite" as part of a nightly ritual - the entire phrase is "Good night, sleep tight, don't let the bedbugs bite." This phrase comes from the time when matresses rested on rope supports on the bed, and you slept better if the ropes were tight instead of sagging. Bedbugs are bugs that crawl around the house after dark looking for blood - they get into the bed and bite whoever is asleep, leaving painful and very itchy sores.