You may have heard about circadian rhythms, the sleep/wake cycles that occur every 24 hours. They’re important to your body’s energy use, but there’s another type of rhythm – ultradian – that has an even bigger impact on energy generation. “Ultradian” refers to any cycle that repeats itself a number of times within the 24-hour period. Every body system, including heart rate, body temperature, digestion, memory and muscle strength, is governed by its own ultradian cycle and associated peaks and troughs. You experience the high points of ultradian cycles as bursts or flows of physical energy, alertness and creativity. You experience the low points as periods of fatigue, distraction and diminished capacity.

“Most people don’t know they have a natural 90- to 120-minute period of energy,” explains psychobiology researcher and therapist Ernest Rossi, PhD, who explored the influence of ultradian rhythms in his book The 20 Minute Break (Jeremy P. Tarcher, 1991). “Research indicates that all our major mind-body systems of self-regulation, including the autonomic nervous system, the endocrine system, and the immune system, have rest-activity cycles.”

After 90 to 120 minutes of peak activity, he explains, the human system goes into an energy dip for approximately 20 minutes, during which you may feel physically fatigued, mentally unfocused, hungry or grumpy. It’s during these dips, says Rossi, that each of the body’s systems replenishes its energy supply at the cellular level.

During an active phase, a cell extracts energy from adenosine triphosphate or ATP, changing it to adenosine diphosphate or ADP. During rest, the cell uses oxygen and blood glucose to change the ADP back to ATP.