Do computers get tired?

I am a Network Admin at a church with about 45 computers. I have heard a few people say throughout my year and a half here, that they feel like their computers are getting slower as the day goes by? Is this possible for a computer to react slower from simply working? or better yet and more likely, what are some things that can cause network slugishness to increase as the day goes by? Or is it that the people are simply more tired and wanting their computer to react faster and it seems slower to them. Personally I have not noticed any increase or decrease on system or network performance. Any ideas?