There is a good possibility that your soul trys to leave your body, even though your not willing it to, when this happens your soul still feels the connection to the body. There for the senation that you get when you try to get up is that your body is still sleeping but you are trying to get up. There for your soul and body feels very heavy. At other times it's succesful and that could be a reason why you feel light. The soul is still connected to the body so there for still sent information to the brain. Of feeling that you are havy or light.

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Immortal we may be but still not out of reach from deaths hand