You say, "This is my body" this indicates that you are different from the body and the body is your instrument. You are holding it just as you hold a walking stick in your hand.
In sleep you exist independent of the walking stick in your hand (body). In dreams you operate through the astral body without having any concern for the fleshy body. Through ignorance you have identified yourself with the physical body and mistaken it for the real 'I' which is everpure, allpervading, selfexistent, selfluminous and selfcontained, which has neither beginning nor end, which is changeless, beyond time, space and causation, and which exists in the past the present and the future.
Prana (vital force) is not 'I'. It is the effect of rajas (energy). It is inert. It cannot welcome a man while you are asleep, though it is flowing. It increases and decreases. You say, "My prana" this shows you are different from prana. It is your instrument only. You can control the breath by pranayama. The controller is different from the controlled (prana). Prana is not 'I'.
Mind also is not 'I'. It gropes in darkness. It borrows light from a higher power. It gets puzzled and confused. During shock and fear it becomes insentient. It is the effect of satva. It is your instrument. You say, "my mind" therefore mind is different from 'I'. It is full of changing ideas. It has a beginning and an end. You can control the mind and the thoughts the controller is different from the controlled (mind). It is as much your property, and outside of you, as your limbs etc., or dress, chair, etc. In sleep there is no mind yet you wake up with a feeling of continuity of consciousness. There is no mind in delirium or coma yet 'I' remains. Mind is a bundle of thoughts and all thoughts are centred around the false egoistic little 'I'. The root thought of all these thoughts is the 'I' that is full of vanities.
Talking of myself, I always speak of 'I'. The sheaths in which I am happy, old, black, a sanyasi (monk), etc., are incidents in the continuity of the 'I'. They are ever changing and varying but the 'I' remains the same unchanging amid the changing.