Lately I can't sleep at night because of work-related worries.
He seems to have something on his mind and has been silent the whole time.
deep in thought; absorbed in thought
Describes the state of being lost in thought, often without a specific object of worry. Common in phrases like 考え事をする or 考え事にふける.
She was walking while deep in thought and bumped into a utility pole.
I was absorbed in thought and didn't notice when someone called me.