The term "turn in" likely originated from the nautical practice of sailors "turning in" to their hammocks at the end of the day.
* Limited Space: On ships, space was very limited, and sailors' hammocks were often stacked in tiers. To get into their hammocks, they had to literally "turn in" or maneuver themselves into the confined space.
* Metaphorical Extension: Over time, this physical act of "turning in" to a confined space came to be used metaphorically to describe going to bed or retiring for the night, even when not sleeping in a hammock.
This nautical origin explains why "turn in" specifically refers to going to bed, rather than simply going to sle
ep.