What Is the Ultradian Rhythm, and Should You Work in 90-Minute Blocks?
Quick answer
The ultradian rhythm is a biological cycle shorter than a full day, most rigorously documented in roughly 90-minute sleep stages, and later extended by some researchers into the idea that waking alertness follows a similar ebb-and-flow pattern. The popular "90 minutes of focus, then a break" productivity advice is built on that extrapolation, which has genuine appeal but is a looser, less directly tested idea than the sleep-cycle research it borrows credibility from. The broader, better-supported takeaway is that scheduled breaks between focus blocks help, even if the exact 90-minute number is more rule-of-thumb than hard science.
Want to try this yourself? Open the Focus page.
If you've seen advice to work in 90-minute sprints because that's how your brain's "natural cycle" runs, the idea traces back to real sleep research, stretched into a workday productivity rule that goes further than the original science does.
Where the 90-minute figure actually comes from
Sleep researchers have long documented that a full night's sleep moves through cycles of roughly 90 minutes each, passing through lighter and deeper stages, including REM sleep, in a repeating pattern. That part of the research is solid and well replicated. The leap to waking hours came later, when some researchers proposed that a similar roughly 90-minute rhythm of higher and lower alertness might continue through the day, an idea usually called the basic rest-activity cycle.
Why the workday version is shakier ground
The waking-hours extension of this idea has nowhere near the same depth of controlled research behind it as the sleep-stage findings it's inspired by. It's a plausible, appealing hypothesis, attention and energy probably do naturally rise and fall rather than staying constant, but the specific claim that everyone's cycle runs exactly 90 minutes, and that working in exact 90/20 blocks will reliably boost performance, outpaces what's actually been rigorously tested.
What's worth keeping from the advice anyway
Even without the precise number holding up perfectly, the core behavior the advice encourages, working in defined blocks with real breaks in between rather than pushing through fatigue for hours unbroken, has broader support in fatigue and attention research generally. Treating 90 minutes as a reasonable starting point to experiment with, rather than a fixed biological law, is the more honest way to use the idea.
Our Focus page lets you set a custom work interval instead of defaulting to a rigid Pomodoro count, so you can test whether a longer block actually suits your own attention span better than 25 minutes does.