how come "turn in" means go to sleep?

Usage of “turn in” as an idiomatic expression meaning “go to bed” dates back to the 17th century and its origin appears to be from nautical jargon:

Turn in:

1. to go to bed.

1695 [UK] Congreve Love for Love III i: I mean to toss a can, and remember my sweetheart afore I turn in.

1751 [UK] Smollett Peregrine Pickle (1964) 584: You sister Mrs. Clover keeps close watch upon her kinsman, without ever turning in.

1767 [US] ‘Andrew Barton’ Disappointment II i: Let’s step into the state-room and turn in.

(Green’s Dictionary of Slang)

Unluckily GDoS doesn’t explain how turn in come to mean “go to bed”, but one suggestion comes from The Phrase Finder

I believe the space on old-times boats was quite limited and the sailors' beds (most probably of hamac-type in two tiers) had a very restrictive clearance to accommodate a person (which is the case even nowadays on small boats). Therefore the person to go to bed had to literally "turn in".

and also Etymonline suggests a possible nautical origin

Turn in:

American English. Turn in "go to bed" is attested from 1690s, originally nautical.

and according to A Dictionary of Slang and Unconventional English turn in:

1695 (Congreve) colloquial nautical till mid 19th century, then a general usage.