Sleep: Tight

If the ropes were tight, the bed was flat, firm, and comfortable. Thus, to "sleep tight" originally meant to sleep on a properly tensioned rope bed. If the ropes went slack, you woke up with a sore spine, essentially sleeping in a hammock of despair.

Etymologists often point to an older meaning of the word "tight," which meant "soundly" or "securely". In this sense, to sleep tight is simply to sleep deeply and well. Why Sleeping Tight Matters Sleep Tight