Ultradian Rhythms and the 90-Minute Focus Cycle

There's a specific kind of tired that hits in the middle of a work session that has nothing to do with how much sleep you got. You were completely focused twenty minutes ago, and now you're reading the same sentence over and over. More caffeine won't fix it.

What is it?

In 1963, Nathaniel Kleitman, the same researcher who co-discovered REM sleep, proposed that the 90-minute rhythm governing your sleep stages at night doesn't stop when you wake up. It keeps running all day long. He called it the Basic Rest-Activity Cycle, or BRAC. Every 90 minutes or so, your brain moves through a full arc of high alertness and focused capacity, followed by a trough of reduced arousal where it needs to recover.

During the active phase, cognitive performance is sharp, working memory is strong, and sustained attention comes relatively easily. As you approach the trough, usually around the 80 to 90 minute mark, brain wave activity slows, norepinephrine and dopamine levels dip, and focus becomes effortful. Most people reach for more willpower at this point. The biology is asking for a break.

The Neuroscience

When you push past the trough instead of resting, your brain doesn't simply stay at reduced capacity and wait for you to catch up. Cortisol rises as a compensatory mechanism. Error rates increase while your awareness of those errors decreases, which is a particularly costly combination. The quality of your output drops faster than your perception of it does, which means grinding through exhaustion often feels more productive than it actually is.

Ericsson, Krampe, and Tesch-Römer's 1993 study on elite violinists turned up something worth paying attention to. The best performers didn't practice more total hours than average performers. They practiced in focused sessions of roughly 60 to 90 minutes, took genuine breaks between them, and rarely exceeded four hours of deliberate practice per day. The constraint wasn't motivation or talent. It was biology, and they'd learned to work with it.

The Pomodoro Technique, 25 minutes of work followed by a 5-minute break, became popular because it gets people to stop treating focus as a continuous resource. The issue is that 25 minutes often cuts the active phase of an ultradian cycle short. For tasks requiring deep cognitive engagement, 25 minutes is just long enough to reach genuine focus before the timer interrupts. A better fit for most people is 60 to 90 minutes of focused work, followed by a genuine 15 to 20 minute break. Not a phone-scroll break. A proper rest: a walk, some water, a few minutes of letting your mind wander. The default mode network activates during downtime and consolidates what you've been processing. That rest is part of the work.

Try it this week

Start paying attention to when your focus naturally dips. Once you know what to look for, the signals are easy to spot. Re-reading the same line multiple times, restlessness, yawning, or a sudden strong urge to check your phone are all signs the trough has arrived. When those signals appear, stop for 15 to 20 minutes instead of pushing through.

Try one 90-minute focused work block today. No switching, no phone. Then take a real break before starting the next one. Schedule your hardest cognitive work in the first block of the day, when your brain's capacity is at its highest.

Benji Lachar

High schooler from Dallas, Texas, writing about the neuroscience behind habits, learning, and everyday life at softreset.blog.