When bed time rolls around your body temperature naturally begins to fall which is preparing your body for sleep.
During the night your body temperatures falls even lower naturally which helps keep your body in sleep and by the morning your body temperature once again rises to prepare you for being fully awake.
When our bodies are cooler we sleep better and when our bodies are hotter we tend to have trouble sleeping so it's important for our bodies temperature to fall to a natural low temperature at night and then return to normal daytime body temperatures in the day.