We human beings are designed to move between spending and renewing energy. We’re at our best when we align with our internal rhythms. That means sleeping at night and being awake during the day. At night, we sleep in something called the Basic Rest Activity cycle—five stages, from light to deep sleep and back out again approximately every ninety minutes. This same cycle recapitulates itself during the day, except we move every ninety minutes from high physiological alertness progressively down into a state of fatigue.
