90/20 deep work timer
The 90/20 deep work timer runs 90-minute focus blocks with 20-minute breaks, and a 30-minute break after every second block. The 90-minute block is the longest on Freemato apart from the 2-hour timer, and it is built for work where the first 30 minutes are just getting the problem back into your head.
The 90-minute figure is usually traced to Nathaniel Kleitman's work on ultradian rhythms, the idea that the body runs on cycles of roughly 90 minutes through the day as well as through sleep. Applied to attention, that is a hypothesis, not an established fact. It has not been shown that focus reliably fades at minute 90. Treat the number as a reasonable ceiling for one sitting rather than a biological law.
This preset suits researchers, developers, and writers who already know they can work for an hour without looking up and want a timer that does not interrupt them before they are done. Do not use it if you are new to timed work or if you are tired, because 90 minutes is a long time to fail at, and a block you abandon at 40 is worse than a 40/10 you finish.
Questions
is 90 minutes too long for a pomodoro
For the classic technique, yes. Cirillo's method uses 25-minute blocks. A 90-minute block is a different tool for a different job: deep, single-task work where interruptions cost a lot. If you drift at 45 minutes, this is not the timer for you yet.
what is the ultradian rhythm and does it apply to focus
It is the idea, from Nathaniel Kleitman's research, that the body cycles through roughly 90-minute phases during the day, not just during sleep. Whether attention follows the same cycle is a hypothesis. It has not been established for focus.
how many 90 minute blocks can you do in a day
2 blocks is 3 hours of deep work, with a 20-minute break between them and a 30-minute break after the second. 2 is the realistic target for most days. A third works if the first 2 went well.