The 90-Minute Pulse: Ultradian Rhythms and the Structure of Attention

You've experienced it even if you don't have a name for it—about ninety minutes into focused work, attention wavers. The mind drifts. The body wants to move, stretch, look away. Push through, and productivity drops. Take a break, and focus returns.

This isn't a weakness or a distraction. It's biology.

Beneath the circadian rhythm—the 24-hour cycle that governs sleep and waking—runs another pattern: the ultradian rhythm, cycling roughly every 90 to 120 minutes throughout the day. Sleep researchers first identified it in the stages of nighttime sleep. Later research found the same rhythm operating during waking hours.

The organizational implications are significant, and almost entirely ignored.

The Rhythm

Nathaniel Kleitman, the "father of sleep research," discovered that sleep cycles through stages in approximately 90-minute intervals—from light sleep through deep sleep through REM and back again. This pattern repeats four to six times per night.

Kleitman hypothesized that the same rhythm continued during waking hours, a concept he called the Basic Rest-Activity Cycle (BRAC). Subsequent research has largely confirmed this: humans operate in roughly 90-minute cycles of higher and lower alertness throughout the day.

During the "up" phase of the cycle, cognitive capacity peaks. Attention focuses more easily. Complex work flows. During the "down" phase, the brain seeks rest—consolidation, recovery, diffuse processing. Fighting this phase is possible but costly.

The precise timing varies by individual and context. Some people run closer to 80 minutes; others closer to 120. But the pattern appears to be universal: waves of engagement and disengagement, roughly every hour and a half.

The Organizational Mismatch

Most organizations structure time as if attention were constant—available in uniform blocks to be scheduled without regard to biological rhythm.

The hour-long meeting default. Calendar systems default to 60-minute blocks. But 60 minutes doesn't align with natural attention cycles. A 90-minute meeting might allow completion of a full cognitive cycle; a 60-minute meeting interrupts mid-cycle; a 30-minute meeting barely begins one.

The eight-hour assumption. The workday is treated as eight hours of equivalent capacity. But if attention runs in 90-minute waves, an eight-hour day contains perhaps five full cycles—not eight hours of constant productivity. Scheduling as if all hours are equal ignores the wave structure.

The back-to-back pattern. Meeting-dense calendars often stack commitments without breaks. If each meeting runs 60 minutes, and they're scheduled consecutively, there's no accommodation for the low phase of the cycle. People arrive at afternoon meetings already depleted.

The break stigma. Taking breaks—real breaks, not checking email—is often seen as unproductive. But breaks may be what the ultradian rhythm requires. The person stepping away after 90 minutes of focus isn't slacking; they're following biology.

What We're Observing

When we look at organizational time through the lens of ultradian rhythms, certain patterns become visible:

The afternoon collapse. Energy and attention often crater in the early-to-mid afternoon. This is partly circadian (the post-lunch dip), but also ultradian accumulation: by 2 PM, people have fought through multiple cycles without adequate recovery. The deficit compounds.

The meeting marathon effect. Days packed with meetings leave people more exhausted than days with equivalent hours of solo work. One hypothesis: meetings impose external structure that prevents natural rhythm-following. You can't take a break when you're in a meeting.

The creative timing pattern. Many people find creative work easier in the morning, not just because of alertness but because they haven't yet accumulated ultradian debt. The first cycle of the day is often the cleanest.

The break quality question. Not all breaks are equal. Checking email doesn't provide the recovery an ultradian trough requires. Neither does switching to another cognitively demanding task. The rhythm seems to call for actual rest—movement, nature, unfocused attention.

The deep work structure. People who produce sustained creative or intellectual work often structure their days around something like 90-minute blocks—whether or not they know the research. They've discovered empirically what the rhythm requires.

The Design Implications

If ultradian rhythms are real—and the research suggests they are—then organizational time design has been working against biology.

Meeting duration. The 60-minute default is arbitrary. Meetings might be designed around complete cycles (90 minutes for deep work sessions) or partial cycles (25-30 minutes for updates that don't require sustained attention).

Break architecture. Rather than treating breaks as lost time, they might be designed into the day as rhythm-recovery. The Pomodoro Technique (25 minutes on, 5 minutes off) is one approximation; 90 minutes on, 20 minutes off might be another.

Calendar structure. Instead of filling every available slot, calendars might be designed with rhythm in mind—blocks of focused time followed by genuine recovery, rather than continuous commitment.

Energy mapping. People might track when their attention peaks and troughs, and schedule accordingly. High-cognitive work in peak phases; administrative work in troughs. This is personalized rhythm-following.

The Tension

There's a real tension between ultradian rhythms and organizational coordination. Meetings require synchronization across multiple people's calendars. If everyone optimized for their own rhythm, coordination would collapse.

But the current default—ignoring rhythm entirely—has costs that are real if invisible. Depleted attention, afternoon crashes, meeting fatigue, creative blocks. These aren't personal failings; they're predictable consequences of rhythm-ignoring design.

We don't yet know the optimal balance between individual rhythm-following and collective coordination. But we're observing that organizations which build some accommodation for biological rhythm—protected focus blocks, sanctioned breaks, awareness of energy patterns—seem to get more from their people than those that treat attention as constant and schedulable.

What to Look For

In your own context, you might observe:

• When your attention naturally peaks and troughs through the day
• Whether your meeting load allows for rhythm-recovery or forces continuous performance
• How you feel at the end of a meeting-heavy day versus a focus-heavy day with breaks
• Whether your best work happens in the first 90 minutes of focus or later
• What kind of break actually restores your capacity versus what just fills time
• Whether organizational culture supports breaks or stigmatizes them

The question ultradian rhythms raise isn't "how do we get more hours?" It's "how do we work with the natural structure of attention rather than against it?"

That's a different kind of temporal design—one that treats human biology as a constraint to respect rather than an obstacle to overcome.

Temporacy is investigating the hidden temporal structures that shape organizational life. Ultradian rhythms offer one lens for seeing what's usually invisible: the wave structure of attention that runs beneath the surface of the workday. We're exploring what this means for organizations designing how time is used.