Guide

Ultradian rhythms and the 90-minute cycle.

Your focus is not a flat line you can hold all day. It rises, peaks, and dips in waves, and each wave runs about 90 minutes. Work with the wave and you get more done with less strain. Fight it and you burn out by lunch.

What is an ultradian rhythm?

A circadian rhythm is the 24-hour cycle that governs sleep and waking. An ultradian rhythm is shorter. It cycles more than once a day. Your body runs many of these, from hormone release to digestion, but the one that matters most for work is a roughly 90-minute cycle of alertness. Your brain climbs toward a peak of sharp focus, holds it for a while, then slides into a trough where attention scatters and you start reaching for your phone.

That trough is not a character flaw. It is a signal. Your brain is asking for a short recovery before the next climb.

What is the science behind it?

The idea traces back to Nathaniel Kleitman, a pioneer of sleep research. While studying sleep cycles, he noticed the brain runs through stages on a roughly 90-minute loop. He proposed that this same cycle, which he called the basic rest-activity cycle, or BRAC, keeps running while you are awake. During the day it shows up as alternating periods of higher and lower energy rather than sleep stages.

Later researchers and performance writers, including Tony Schwartz and Jim Loehr in their work on managing energy, built on this to argue that the body is meant to pulse between effort and recovery, not push at full output for hours. The practical takeaway is the same across all of them: roughly 90 minutes of focus, then a real break.

How do you schedule deep work around it?

1. Find your peak windows. Most people get a strong cycle in the late morning and another in the late afternoon. Watch yourself for a few days and note when focus comes easily.
2. Put your hardest work in a peak. Schedule your most demanding deep work to start at the front of an up-wave, not after lunch when you are dipping.
3. Work one cycle, then stop. Run about 90 minutes, then take a genuine 15 to 20 minute break. Move, look away from screens, get outside if you can.
4. Do not push through the dip. When focus drops, that is the rest part of the cycle. Grinding past it gives you slow, error-filled work. Resting resets you for a clean next cycle.
5. Save shallow work for the troughs. Email, filing, and easy admin fit naturally into the lower-energy parts of the day.

How does this connect to a 90/15 timer?

A 90/15 round, meaning 90 minutes of work followed by a 15-minute break, is the ultradian rhythm turned into a timer. One round equals roughly one full cycle. You ride the wave from the climb through the peak, then break before the trash-attention trough sets in. This is why 90/15 suits deep creative work when you are fresh, while shorter rounds like 25/5 or 50/10 suit scattered energy or admin.

Frequently asked questions

---

Keep reading