These ultradian rhythms help to account for the ebb and flow of our energy throughout the day. Physiological measures such as heart rate, hormonal levels, muscle tension and brain-wave activity all increase during the first part of the cycle—and so does alertness. After an hour or so, these measures start to decline. Somewhere between 90 and 120 minutes, the body begins to crave a period of rest and recovery. Signals include a desire to yawn and stretch, hunger pangs, increased tension, difficulty concentrating, an inclination to procrastinate or fantasize, and a higher incidence of mistakes.
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