" We have body clocks not just in the brain but all over—in our pancreas, liver, heart, kidneys, fatty tissue, muscle, virtually everywhere—and these operate to their own timetables, dictating when hormones are released or organs are busiest or most relaxed. Your reflexes, for instance, are at their sharpest in mid-afternoon, while blood pressure peaks toward evening. Men tend to pump more testosterone early in the morning than later in the day. "

― Bill Bryson , The Body: A Guide for Occupants