What is ultradian rhythm 90-minute cycle Kleitman?

Ultradian rhythm 90-minute cycle Kleitman is a concept in personal epistemology: Some days you have more capacity than others — plan for this variability.

Example: You commit to four hours of deep work every day. Monday you hit four hours and feel sharp. Tuesday you slept poorly after a late flight, your back aches, and an unresolved argument sits in the back of your mind — you grind through two hours of low-quality output and abandon the other two. You mark Tuesday as a failure. Wednesday you try again, hit three hours, and mark it as another failure. By Friday you have "failed" three of five days and conclude you lack discipline. But your actual weekly deep-work total was fifteen hours — a strong number. The problem was never your output. The problem was that you measured every day against a fixed target instead of recognizing that capacity fluctuates and planning accordingly. A variable system judged by a constant standard will always appear broken.

This concept is part of Phase 49 (Capacity Planning) in the How to Think curriculum, which builds the epistemic infrastructure for capacity planning.