Ultradian Rhythms: Why Focus Comes in Roughly 90-Minute Waves
You are not designed to run at a constant level all day. Alertness rises and falls in waves, and one of those waves is roughly 90 minutes long. Understanding it helps explain why you can be razor-sharp at 10 a.m. and foggy by noon — and how to work with the rhythm instead of against it.
The Basic Rest-Activity Cycle
The idea traces back to the sleep researcher Nathaniel Kleitman, who spent his career mapping the architecture of sleep. He noticed that sleep is not uniform: it cycles through lighter and deeper stages, including REM, on a period of roughly 90 minutes. Kleitman proposed that this same clock keeps ticking while we are awake — a Basic Rest-Activity Cycle (BRAC) that swings us between higher and lower alertness across the day, not just at night.
The term "ultradian" simply means a rhythm with a period shorter than a day (as opposed to the roughly 24-hour circadian rhythm that governs your sleep-wake timing). Later researchers, including Peretz Lavie, found ultradian fluctuations in daytime alertness and the tendency to fall asleep, giving the waking BRAC some empirical support.
It is worth being honest about the state of the evidence: the nighttime ~90-minute sleep cycle is well established, while the strength and exact length of a strict daytime cycle vary between people and studies. Think of "90 minutes" as a useful rule of thumb, not a precise metronome ticking in your skull.
What it feels like in practice
If you pay attention, you can often sense the wave. There is a window where work feels almost frictionless — ideas connect, distractions bounce off. Then, usually after somewhere between 60 and 120 minutes, a trough arrives: you reread the same sentence, you get restless, you suddenly need a snack or your phone. That dip is not a character flaw. It is the bottom of a cycle, and it is a signal.
Most of us try to power through the trough with more coffee and more willpower. That works for a while, but pushing hard through the low point tends to produce diminishing returns and mounting mistakes — the vigilance decrement stacked on top of a natural energy dip.
Working in waves
The practical translation is simple: ride the wave up, then recover — do not fight the trough.
- Block deep work in roughly 90-minute sessions. Pick your single hardest task and give it one uninterrupted wave, ideally in the morning when alertness tends to be highest for most people.
- Break at the trough, not through it. When focus starts to slip, treat it as the end of the cycle. Step away for 10 to 20 minutes rather than grinding for another hour at half speed.
- Make the recovery genuinely restorative. The goal is to let your attention system reset: move your body, get some daylight, breathe slowly, hydrate, or simply rest your eyes. A break that is just a different screen does not recharge the same battery.
- Stop counting blocks by mid-afternoon. Later cycles are shallower. Schedule easier, lower-stakes work for the afternoon and protect the mornings for what matters most.
How this differs from Pomodoro
A 25-minute Pomodoro and a 90-minute ultradian block are not rivals — they operate at different scales. Pomodoros are great for getting started and for shallow or aversive tasks, where the short boundary lowers the activation cost. Ultradian blocks suit deep work that needs a long runway and rewards staying in flow. Many people nest the two: a single ~90-minute wave of focused work, with one short stretch-and-breathe break in the middle, followed by a real recovery break at the trough.
Mapping your own waves
Because the exact timing varies so much from person to person, the most useful thing you can do is measure your own pattern rather than trust a generic number. It takes about a week and no special tools:
- Rate your energy hourly. Set a quiet reminder each hour and jot a 1-to-5 alertness score plus a word or two about what you were doing. After a few days, peaks and troughs start to show a rough schedule.
- Watch for the "restless" tell. The trough often announces itself the same way each time — rereading, fidgeting, a craving for a snack or your phone. Learn your personal signal and treat it as the cue to break rather than to push.
- Note when your best ideas land. Many people have one or two windows a day when hard thinking feels easy. Those are the blocks to guard for your most demanding work.
Once you know your shape, build the day around it: hardest task in your top peak, meetings and admin in the shallows, and a real break booked at each predictable trough.
A caveat worth keeping
Your rhythm is your own. Chronotype (whether you are a morning lark or a night owl), sleep debt, caffeine, stress, and the task itself all shift where your peaks and troughs land. The lesson from ultradian research is not "work in exactly 90-minute blocks" — it is "energy is cyclical, so schedule your hardest work for your peaks and build in real recovery at your troughs." Notice your own pattern for a week, and pace the day around it.