Why does physical pain intensify at night?

I’ve noticed upon having a sports injury (pulled shoulder muscle), that the pain intensifies when the sun goes down. During the day, I do not need to take any medicine for the pain, but upon night fall, it hurts to the point that I need the medicine. Why is this? Also, if I have a cold, the symptoms are usually bearable during the day, but at night they worsen. Why is this?