How do I keep my memory foam mattress from absorbing heat?

I love my memory foam mattress, but this is my first summer with it and the amount of heat it absorbs throughout the night is unbearable. How can I prevent my mattress from absorbing all of that heat?
Update: I come from a green family and cannot install an air conditioner due to my parents wishes and my bed is not in the direct sunlight. It is only a problem at night when it absorbs my body heat.
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