When you sleep your body goes through cycles, typically when you are hitting deep sleep your body becomes cooler. However if your body becomes to hot during this period your sleep will not be as deep and you will feel more fatigued the following day.
Sleeping on a foam or synthetic fiber mattress can trap in body heat which will build with an oven like effect. What usually happens is people will push of the covers when they become too hot, but then they will eventually become too cold this will interupt your sleep even more. The key to good sleep is conistant temperature. Sleeping on natural fibers will draw the heat away and redisperse it keeping your body at optimal sleeping temperature. Not only this but natural mattresses tend to last longer.