I can't hold my head up

I'm sooooooooooo tired, I can barely focus my eyes. I just want to put my head on the desk and have a nap. I felt like this at work last week too, but not over the weekend/ice days at home. Which makes me think that it's related to riding to work. However, I'm not going to stop riding to work because I feel very sleepy mid-afternoon, it's not a good enough reason. But I'm sooooo tired.