Recalling the fitt principles, decide which principle is used when you continuing to exercise regularly so that you keep the benefits you have gained from exercise.

F- frequency
I- intensity
T- time
T- type
Frequency is how often/frequent you exercise or train.

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