Working in waves: ultradian rhythms and the 90-minute question

Some hours the work pours; some hours the same task feels like wading. Productivity culture explains the difference as discipline. Physiology has a humbler explanation: alertness moves in waves, and you were in a trough. The interesting question is not how to abolish the trough — you cannot — but how to stop scheduling your hardest work inside it.

What is actually established here

The term "ultradian rhythm" covers biological cycles shorter than a day, and the nighttime version is beyond dispute: sleep runs in roughly 90-minute cycles through the night — that part is textbook, and we leaned on it in sleep debt. The daytime claim — that waking alertness also oscillates on a rough 90-to-120-minute period — has real research behind it going back to sleep scientist Nathaniel Kleitman, who proposed a "basic rest-activity cycle" continuing around the clock.

Honesty requires splitting the claim in two. That daytime alertness fluctuates in cycles: well established. That your cycle is exactly 90 minutes and you should architect life around that number: early evidence at best — the daytime research is thinner and messier than the productivity books imply, and the period varies by person and by day. The waves are real; the stopwatch is marketing.

The expensive habit: fighting the trough

The trough has a recognizable signature — rereading the same paragraph, drifting to the browser tab, a sudden fascination with the kitchen. The standard response is to push through with willpower and caffeine, which works the way overdrafts work: it borrows from later at a fee. Some troughs are worth pushing through; a deadline is a deadline. But defaulting to the push on every dip means spending your most expensive fuel on your least productive minutes — and the 3 p.m. crash collects the fee with interest.

The cheaper response, when the calendar allows: ride it. Ten or fifteen minutes of genuinely different activity — a walk, water, a low-stakes chore, anything that is not the screen — and the next wave tends to arrive on its own. Not a reward, not a failure. A tide.

Finding your waves without buying anything

Skip the wearable; run the paper version. For one week, jot a number from 1 to 5 for alertness every hour or so — ten seconds, no analysis. By Friday most people can see their shape: where the strongest stretch reliably lands, where the trough does. The finding is usually boring and extremely useful: one or two genuinely sharp windows a day, which the current schedule was spending on email.

Then the move is obvious — put the one task that deserves a clear head inside the strongest window, and defend that hour like it is worth something, because it is. That is the entire thesis of the Clear Hour system, and the Focus Planner has the hourly log pre-built so the week of jotting has somewhere to live.

FAQ

Should my work blocks be exactly 90 minutes?

No — the number is a folk average, not a prescription. Work until the signature of the trough shows up, then break properly. Some days that is 50 minutes; some days it is two hours.

Is the afternoon dip an ultradian trough?

Mostly no — the early-afternoon dip is circadian, a once-a-day scheduled sag that lands whether or not you had lunch. Ultradian troughs repeat through the day on top of it. Same signature, different clock, same advice.

Where does caffeine fit?

As a scheduled tool, not a trough-eraser — caffeine on every dip is how tolerance builds and afternoons unravel. The arithmetic is in the caffeine cut-off.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Educational content, not medical advice.