I thought the sum total of mass/energy in the universe was constant, if you travel back in time you would be adding to this total.
Each molecule in your body would already be in use, ie the water molecules could be spread out in rockpools or combined into the bark of a tree somewhere, the trace iron in your blood could be thousands of feet beneath the earth.
The only way to do it would be to transfer a equal amout of mass /energy forward to your time at the same instant you left.
Think time travel is a no no.