Meaning of "turn in"

[Anchela]

English Language Tests, Intermediate level

ESL/EFL Test #207 "What Comes Next? (1)", question 2

I'm very tired so I think I'll ..........

(a) go out
(b) turn off
(c) turn in
(d) turn over

English Language Tests, Intermediate level

ESL/EFL Test #207 "What Comes Next? (1)", answer 2

I'm very tired so I think I'll turn in.

Correct answer: (c) turn in

Your answer was: incorrect
I'm very tired so I think I'll turn off.
_________________________

I don't understand the meaning of this sentence.
What does "turn in" mean?

Anchela

[Conchita]

'Turn in' is an informal way ot saying 'go to bed'.