Having stood up for almost a week solid, I would have taken any excuse to end my self imposed torture, and this seemed like a valid reason at the time. In hindsight, the evening and night were the danger times.
And so it was that I found myself in my desk chair at three in the morning, nodding off. I think that I only slept for about 10 minutes, but that is an eternity when you only sleep for 20 minutes during a full nap. It's hard to explain how furious I was with myself.
It's said that an oversleep during the initial few days (up to day 30) sets you back by about 50% of the time you have done so far. In my case, that would effectively wipe out the last five days. Hard won days, at that. I beat myself up for my oversleep for the next four hours. On the positive side, this did keep me awake and occupied for that phase of the day.