This song could potentially be about a person suffering from schizophrenia, who is friends with a imaginary person. He is certain that this person is real, and it keeps him up at night.
See-see
I can't see
What you're trying to do to me
Oh no
Oh no
Oh yeah
Cause you're in my head
So I never go to bed oh no
Oh no
Oh no
Talk-talk, it's all you do
So I never talk to you oh no
Oh no
Oh yeah
Cause you're in my head, so I never go to bed, oh no
Oh no
Cause you, you are an imaginary person
You're in my head but I am certain, you are real
You are, an imaginary person
You're in my head but I am certain, you are real
Yeah, you are real
You are real
You are real
You are real