I've been told that it's a good idea to track the miles per gallon that your car gets over time. That way, if you suddenly start getting terrible gas mileage, you'll know that something's wrong with your car.
When you have a physical ailment of some kind, the first thing a doctor or webpages will ask is, "how long has it been like this?" If you haven't been keeping track, you'll probably only have a vague idea.
Several times in the last few years, stress-induced ailments have pushed me to the point of wanting help. But, since I didn't have a baseline, I had no idea when it started "getting bad" only that it was bad right then.
To remedy this, I've started taking notes in ridiculous detail. I've been using Evernote for this because it's free and has Mac and Windows clients. I went looking for an iPod Touch app for this, but they were either too specialized or too expensive. Other than feeling kind of silly for taking the notes, it's working out okay.
With luck, the next time I need to know how long I've been sleeping poorly, I'll have a record of it.