And indeed, if I were to place electrodes on your head, I could show you this reliable drop in your brain's physiological alertness that happens each and every afternoon. And what this means is that us humans may not have been designed to sleep in a single bout at night, which is what we call monophasic sleep. Rather, we may have been designed to sleep in two bouts. In other words, one longer bout at night and then a siesta-like nap in the afternoon.