In my anecdotal experience, if you don't get enough deep sleep for a sustained period of time, you start showing minor cognitive defects - e.g mixing up words, not being able to write long, irritability. Tracking sleep and 'catching up' makes all this go away.
This is especially important for people who are caffeine sensitive, but drink a lot of caffeine. You end up getting 5-6 hours of sleep nightly, and have this constantly mounting cumulative sleep deficit.