Immortality: "It is impossible to be conscious of being unconscious.”
It is not possible to be aware of being unconscious from your own perspective. You cannot be aware of not being aware. You can be less aware/conscious, such as when you are asleep, but not completely unconscious (dead), because time would stand still for you. A billion years could pass, and you would not know it.
How do you know you are dead? It is not possible to be aware of any gaps in life; it is continuous and never-ending from your own point of view.