Ultradian Rhythms
In 1953, Nathaniel Kleitman — the sleep researcher who co-discovered REM sleep — identified a second rhythm in the human body. Beyond the 24-hour circadian cycle, the body moves through roughly 90-minute alternating periods of high and low neural activity. He called these Basic Rest-Activity Cycles (BRAC).
This pattern does not disappear when you wake up. It continues through the day.
What Happens at the 90-Minute Mark
At the end of each ultradian cycle, the body signals a need for rest: yawning, difficulty focusing, a pull toward distraction. Most people override these signals with caffeine or willpower.
The override works temporarily. But sustained suppression degrades performance and compounds fatigue across the day.
Working With the Cycle
- Focus for 90 minutes, then stop deliberately. A five-to-ten minute break — a walk, a stretch, a quiet moment — resets the cycle and prepares the brain for the next block.
- Do not check your phone during the break. The recovery requires genuine cognitive rest, not lateral stimulation.
- Two to three complete cycles per day is sustainable. More is possible but comes at a physiological cost.
Applying This to Your Calendar
- Schedule focus blocks in 90-minute slots, not 60 or 120
- Put buffer time after each block, not just between meetings
- Protect at least two complete cycles per day from interruption
Your most important work should happen in your first complete cycle, when the biological starting point is highest.