Concept
Ultradian Rhythm
An ultradian rhythm is a cycle shorter than a day. Alertness rises and falls in roughly ninety-minute waves, which is why sustained focus arrives in stretches of about that length and why a break after one is not weakness but timing.
The sleep researcher Nathaniel Kleitman, who identified the stages of sleep, proposed that the same roughly ninety-minute cycle continues through the waking day as a basic rest-activity cycle: a stretch of higher alertness followed by a dip in which attention wanders, the body asks for movement, and errors climb. The evidence for exact lengths is looser than popular accounts suggest, and the cycle varies between people and days, but the pattern is real enough to plan around: concentration comes in waves, and fighting the trough with caffeine and willpower buys little. Chris Bailey uses this in Hyperfocus to argue for working with energy rather than clock time: put the hardest work in the peaks, take real breaks in the troughs, and notice your own pattern instead of assuming the textbook one. The practical unit that falls out is the ninety-minute block followed by a genuine pause.
A real-life example
A software engineer in Bursa tracks her alertness hourly for two weeks on a sticky note: sharp from 8:30 to 10:00, flat by 10:30, sharp again around 11:15. She moves her hardest work into the two peaks, takes a walk at 10:15 instead of a third coffee, and stops scheduling design reviews for 14:00, when her notes say she agrees to anything.
How to use it
- 1Rate your alertness each hour for two weeks; the pattern you find is yours, not the textbook's.
- 2Put the hardest work in your peaks and real breaks — movement, not a screen — in the troughs.
- 3Work in blocks of about ninety minutes and stop at the dip rather than pushing through it.