The question Who am I? cannot be explained so easily. You have a name, Bose, Chatterjee, Ramani, whatever, but that is only an external name. You are not that. You need to be called by a name, so you have been given a name, but you are not that. This body, which is called Swami Satyananda, has a limited use, but I am not Swami Satyananda. Inside this body of five elements, ether, wind, fire, earth and water, is the mind. Inside the mind is buddhi, higher intelligence, and inside the buddhi is atman, soul.
A seed contains a tree, but the tree cannot be seen inside it. You have to understand the term inside in this way. So this atman, which never sleeps, is inside you. When you say I, I, I you are referring to the atman. In the context of the body, the atman is called jivatma; when it becomes free of the body, it is called paramatma. A mere veil separates the jivatma from paramatma. When the atman is concerned with life and death it is called jivatma, when it is freed of these ties of ignorance, then there is no difference between it and paramatma. They are like water and ice. When water hardens it becomes ice, when it melts it again becomes water. Therefore, the sages say that while you have the body, sing the name of God.
The atman is immortal. It is unborn and unfettered, it is illumined; there is no scope for darkness in it. The flame of jnana is alight in it. The atman is indestructible; the body is destructible. Disease and death are the laws of the body; the atman does not get sick, it is always effulgent. Therefore, Vedanta says, I am not the body, I am not the senses. Here I means atman.