Imagine you're a predator out hunting for food (and Jesse Ventura), but all the regular animals you would eat are nowhere to be found. You spend the entire day looking for food and find nothing. About 16 hours later your brain starts freaking out. It knows that if you can't find food, the jig will most certainly be up. So at this point, your brain doesn't give a tinkerer's damn about sunlight and sleep cycles -- it just wants you to find something to eat, and fast . You stay up well into the night and eventually find some nocturnal prey, devouring it desperately. Your brain (through the food-clock) makes a note of this time and declares it to be your new biological morning.