When people take a rest in/on the bed, do they lie ON or IN the bed?
(In my native language both are optional)
When people take a rest in/on the bed, do they lie ON or IN the bed?
(In my native language both are optional)
If you lie on top of the sheets, duvet, etc, then you are on the bed. If you get under the sheets, then you are in bed, and you can lie in bed, stay in bed, read in bed, sleep in bed, and do all those other lovely things "in bed" as you please.
on a bed

on top of the bed and its bedding.
in a bed
in bed

(source: mirror.co.uk)
surrounded but the bed and its bedding.
To lie on the bed.
To lie in bed.
To lie in the bed.
All the phrases are grammatical, with a difference in meaning.
You use the phrase "on the bed" when somebody is not covered with sheets or blankets. For example:
He lay on the bed (=on top of the covers).
You use the phrase "in bed" when somebody is covered with sheets or blankets. For example:
He lay in bed (=under covers).
You use the phrase "in the bed" when you are referring to a particular bed. For example:
There was no one in the bed (Longman Dictionary).