"Walking had an added benefit: it helped me to think. Nietzsche wrote, "All truly great thoughts are conceived while walking," and his observation is backed up by science; exercise-induced brain chemicals help people think clearly. In fact, just stepping outside clarifies thinking and boosts energy. Light deprivation is one reason that people feel tired, and even five minutes of daylight stimulates production of serotonin and dopamine, brain chemicals that improve mood. Many times, I'd guiltily leave my desk to take a break, and while I was walking around the block, I'd get some useful insight that had eluded me when I was being virtuously diligent."