An anonymous Redditor claiming to be a cognitive scientist posted the following message in r/Biohackers:
As a cognitive scientist, I’ve spent the past few years digging into what helps the brain sustain high performance, not just in the moment, but over months and years.
One pattern keeps showing up: most people push through mental fatigue without realizing it’s part of a natural biological cycle, not a flaw.
The brain follows predictable cycles of alertness and recovery (called ultradian rhythms), typically every 90–120 minutes. When we ignore those dips and power through with coffee, stimulants, etc. we overload the brain’s recovery systems.
Over time, that can reduce cognitive flexibility.
I’ve been working on ways to help people tune into these cycles more precisely (e.g., like tracking sleep, HRV) and found that you can forecast when your brain is primed for deep focus and when it’s better to rest with just a couple minutes of cognitive testing per day.
I’ve been experimenting with ways to track these rhythms more precisely including a tool I’ve been developing that uses games to forecast peaks and dips. It’s been eye-opening to see how much sharper I feel just by syncing my day to my brain’s actual rhythm.
Curious if anyone else here is measuring or tracking anything similar? Any tool recs out there?
Read the interesting conversation: https://www.reddit.com/r/Biohackers/s/5hTOoGNf0w
He built a tool that tracks brain rhythms throughout the day via mini cognitive assessments.
What we can say about it
Most of us treat mental dips as personal weakness and push through with coffee or will-power. Cognitive science suggests those lulls are part of a built-in “ultradian” rhythm, natural cycles of high alertness followed by recovery that repeat roughly every 90-120 minutes.
When we ignore the recovery phase, the brain’s restorative systems stay switched on for too long. Over weeks or months this strain can dull cognitive flexibility, the ability to switch tasks, think creatively, and adapt to new information. In contrast, working with the cycle is simple: plan intensive tasks during a peak, then take a short break (walk, breathwork, light snack) when focus ebbs. Even a two-minute cognitive self-test or reaction-time game can reveal where you are in the rhythm and help you schedule the next block of deep work or rest.
Early sleep researcher Nathaniel Kleitman first described this “basic rest-activity cycle” in the 1980s, noting that the same 90-minute pattern guiding REM and non-REM sleep also shows up, more subtly, while we are awake. Subsequent studies confirm that alertness, reaction time and even mood tend to crest and dip in step with this rhythm.
Syncing daily work to the brain’s own metronome can therefore boost sustained performance and protect long-term mental energy.
Basic rest–activity cycle (Wikipedia)
The entry explains Nathaniel Kleitman’s hypothesis that humans run on a built-in 90-minute “basic rest–activity cycle” (BRAC) that governs bursts of alertness followed by short recovery phases. It notes that the pattern is easiest to see in REM- and non-REM-sleep alternation but probably also shapes daytime focus and fatigue. Later research both questions and supports the idea, suggesting the daytime cycle exists but may be driven by mechanisms different from those regulating sleep stages.
Kleitman N. (1982) “Basic rest-activity cycle – 22 years later,” Sleep
In this landmark review, Kleitman gathers two decades of data and argues that ultradian (sub-24-hour) rhythms, especially a ~90-minute BRAC, are fundamental to central-nervous-system functioning. He proposes that respecting these cyclic highs and lows conserves mental energy, while ignoring them may degrade performance. Although largely theoretical, the paper set the stage for modern work–rest scheduling based on brain rhythms.
Wiłkość-Dębczyńska M., Liberacka-Dwojak M. (2023) “Time of day and chronotype in the assessment of cognitive functions,” Postępy Psychiatrii i Neurologii
This narrative review shows that cognitive test scores swing with both the time of day and a person’s chronotype (morning- or evening-type). Peak performance happens when testing is in sync with an individual’s internal clock; mismatches can mask true ability or exaggerate deficits. The authors urge clinicians and researchers to control for chronotype and testing hour whenever they assess memory, attention, or executive function.
Complex.so (2025) “How the Ultradian Rhythm Boosts Productivity for Maximum Success”
A popular-science article that translates the 90-120-minute ultradian cycle into a workplace method: tackle high-focus tasks for about 90 minutes, then pause 15–20 minutes for genuine rest (walk, stretch, meditation). The piece warns that powering through low-energy troughs triggers “ultradian stress,” raising cortisol and dulling creativity, whereas cycling work and recovery sustains output and prevents burnout.
Neubauer A.C., Freudenthaler H.H. (1995) “Ultradian rhythms in cognitive performance: no evidence for a 1.5-h rhythm,” Biological Psychology
This controlled lab study tested 60 volunteers every 10 minutes over nine hours with reaction-time tasks, mood ratings, and heart-rate measures. Rigorous spectral analyses found no reliable 90-minute pattern; longer, slower fluctuations explained most variance. The authors conclude that earlier positive reports of a BRAC during wakefulness may reflect statistical artefacts rather than a true 1.5-hour performance rhythm.
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