My theory is that after a long day, you’re more tired and have less energy, and when you’re in that state, you tend to feel “what’s aggravating you” much more strongly. I notice that too, especially with sore throats. On the other hand, some pains for me, like backaches, tend to get better after being awake for awhile——my body has had time to warm up and the muscles have stretched and toned for the evening, so a sore back is usually more stiff in the mornings. But with colds, flu, and most other ailments, the evening is the worse time. And I think it all has to do with your tired state or having less energy and feeling bogged down. The pains are more noticeable, just like when you’re cranky——ever notice how people are more crankier at the end of the day when they’re tired? Even the smallest things can seem annoying. So take a pain and magnify it ten-fold——that’s what you get when you’re tired and worn-out.