Steven Strogatz breaks down the sleep patterns of night owls:
It turns out that the longer you stay up, the less sleep you’ll get afterward. If you haven’t experienced this, it sounds unbelievable, but it’s been confirmed in several experiments and field studies (see the endnotes for references). Every extra hour awake will cost you about 20 or 30 minutes of sleep — assuming you go to bed before noon the next day. The leap occurs if you stay up even longer. Suppose you don’t get to bed till around 3 the following afternoon, having been awake for something like 32 straight hours. How long do you think you’ll sleep now? A 1996 study found that half the subjects woke up after only a lousy little three-hour nap, while the other half corked off for 11 hours!
Let me repeat this since it’s so strange. If you’ve been awake long enough and you fall asleep when your body thinks it’s siesta time, the amount of sleep you’ll get can be either very short or very long. At that time of day, sleep duration jumps abruptly.