"Min son går i sömnen varje natt."

Translation: My son walks in his sleep every night.

[Rebkova]

I do not understand, why is this not translating into "walks in the sleep" (due to sömnen). And therefore why "in his sleep". Wouldn't that translate into "i sin sömn"?

[Lundgren8]

Yes, but the set phrase ’walk in one’s sleep’ in Swedish is gå i sömnen (walk in the sleep). Swedish very often uses the definite form where English uses a possessive pronoun. See also this thread.

[Steph.Ro]

Swedish also often uses the definite form to express "belonging to the subject of the sentence" if it is unambiguous what is meant.

Jag tappade mobilen. - I dropped my phone.

Han hittar inte nycklarna. - He can't find his keys.

Here, for example, the son is extremely unlikely to walk in someone else's sleep. :D

[Gnurfel]

Jag pratar i sömnen men inte varje natt och inte på svenska. Än. :)

[craigchardy]

a somnambule

[susmijor]

Why not translating it correctly into English, e.g. "walks while sleeping"?

[craigchardy]

"Walks in his sleep" or "sleepwalks" IS correct English, while "walks while sleeping" is not idiomatic, at least for American English. See Lundgren8's comment above.