Kleitman and the Basic Rest-Activity Cycle
Nathaniel Kleitman is best known for co-discovering REM sleep in the early 1950s. In the same body of work, he proposed the Basic Rest-Activity Cycle (BRAC), a ~90-minute ultradian rhythm he observed in sleep that he believed also operated during waking hours.
During sleep, the BRAC governs the cycling between non-REM and REM stages, with full cycles running approximately 90 minutes. Kleitman hypothesized that the same oscillation continues in waking life as a 90-minute pattern of higher and lower arousal, alertness, and cognitive capacity, although the physiological signatures are less dramatic than during sleep.
The waking cycle looks roughly like this: the first 60–70 minutes of each cycle correspond to higher alertness, focused thinking, and stronger executive function. The final 20 minutes show a characteristic slowing where brainwave patterns shift, hormone levels trough, and the body signals readiness to transition to the next cycle. Most people experience this as drifting attention, reduced motivation, or a vague restlessness that does not feel quite like tiredness.
A Note on the Science
It is worth being precise about what the research shows and what it does not. The daytime BRAC is well-supported as an observed phenomenon: fluctuations in alertness, reaction time, hormonal levels, and brain activity do follow ultradian patterns. Whether these waking cycles are driven by the same mechanism as the sleep BRAC is still debated; some researchers have argued they operate through different channels.
For practical purposes, the core observation holds regardless of mechanism: cognitive capacity fluctuates in ultradian cycles during the workday, and working in ~90-minute blocks followed by genuine recovery is more aligned with this biological architecture than sustained multi-hour sessions.
Ericsson’s Independent Confirmation
K. Anders Ericsson’s research on expert performance provides an independent data point. Studying elite musicians, Ericsson found that the best performers practiced in sessions of no more than 90 minutes, with clear breaks in between, and capped total deliberate practice at approximately 4 hours per day, roughly four BRAC cycles.
The elite musicians did not cap practice at 4 hours because of preference. Ericsson found that performance and error rates degraded consistently beyond this threshold regardless of the musician’s motivation or dedication. The constraint was not psychological but physiological. The musicians who tried to practice more simply produced lower-quality practice in the additional hours.
What “Pushing Through” Actually Does
The common response to the alertness trough at the end of a BRAC cycle is to push through it: more coffee, more willpower, more time pressure. This works in the short term: stress hormones (cortisol, adrenaline) can override the rest signal and extend cognitive performance past the natural trough. But this comes at a cost.
Regularly overriding the rest signal with stimulants and stress accumulates what sleep researchers call “ultradian debt,” a backlog of incomplete recovery cycles that degrades cognitive baseline over time. The executive who works 12-hour days without genuine rest periods is not doing 12 hours of quality work; they are doing perhaps 4–5 hours of quality work, with the remainder running on fumes while consuming biological credit that shows up as degraded performance the following days.
Practical Structure
The practical implication is not working less. It is working in cycles matched to biological architecture: