Three things the nervous system does

Your nervous system is a system that brings messages about the control of your body. Receiving messages and sending them out, is one of its main jobs. It is made up of three main things, The brain, Spinal cord, and Neurons. 
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Because without physical activity, the body has extra "energy" if you will, and so when it is time for bed you are not tired, so you end up staying up all night. also there is probably an effect on your internal clock, which messes with your melatonin production which goes hand in hand with your sleep cycle