I'm not a doctor, but my level of tiredness seems to vary depending on the ride, how much riding I've done that week, and how much sleep I'm getting. Most people don't realize that hard exercise is also a form of stress and that your body can't differentiate between being buried in too much work at the office or taking a long, hard ride. So, there's a thin line between overdoing it and not when you're stressed out anyway.

Other than that, I make sure to eat right after a ride (4:1 or 3:1 carb to protein ratio), stay hydrated, and make myself stay up and doing stuff because the couch feels way too inviting sometimes!