Research library
Method
Ultradian Rhythms
Kleitman; Schwartz
Cognitive performance pulses in roughly 90 minute waves.
Basic Rest Activity Cycles described by Kleitman extend into waking life. Attention and arousal rise and fall on a 90 to 120 minute cycle. Riding the wave then resting deliberately preserves the next wave.
What the research shows
EEG and behavioral studies show alertness peaks every 90 to 110 minutes followed by 15 to 20 minutes of reduced vigilance.
Where evidence is weaker
Cycle length varies between people and within a day. Sleep debt and caffeine distort the signal. Treat 90 minutes as a useful default, not a law.
How Focus Capsule applies it
The Deep mode preset is 90 minutes of work followed by a 20 minute recovery suggestion. Sprint mode is the shorter Pomodoro variant for fragmented days.