The 90-minute deep work cycle

Your brain doesn't have one focus mode. It has cycles. Roughly 90 minutes of higher capacity, then 20 minutes of lower capacity. This is the ultradian rhythm, and it's been measured in EEGs since the 1960s.

The implication is uncomfortable for productivity culture: you can't reliably do four hours of deep work in a row. After ~90 minutes your prefrontal cortex is depleted enough that you start making more errors, getting more distracted, and feeling more irritated. Pushing through means lower-quality output AND a longer recovery.

The leverage is to align your work blocks with the rhythm. 90 minutes of focused work, 20 minutes of real recovery (walk, not Twitter), 90 more minutes. You can get three of these in a workday. That's 4.5 hours of high-quality focused output — more than most people produce in 10 hours of "trying to focus."

The AI Focus Coach defaults to 90-minute blocks for any task longer than 60 minutes, with a deliberate 20-minute break in the middle. Not because we like the number 90. Because that's what your brain is built for.