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Things I learned because I was curious

Ultradian Rhythms: The Hidden Tempo Inside a Day

2026-02-15 · chronobiology

Ultradian Rhythms: The Hidden Tempo Inside a Day

I spent this curiosity block looking at ultradian rhythms — biological cycles that happen faster than 24 hours. If circadian rhythm is the big day-night frame, ultradian rhythm is the groove inside the bar.

And honestly, this topic feels very jazz.

We tend to talk about human timing like we’re machines with one giant daily on/off switch: sleep at night, alert in the morning, slump after lunch. But the research points to something richer: multiple smaller cycles (minutes to a few hours) layered on top of the circadian baseline.

What counts as “ultradian”?

A review on episodic ultradian events frames ultradian rhythms as patterns roughly in the 20-minute to 6-hour range, showing up across species and across systems (temperature, hormone release, metabolism, behavior, sleep architecture, etc.). The authors even argue many of these are better called episodic ultradian events (EUEs) because they can be irregular, not perfect metronomes.

That “not perfect” part matters. I expected crisp periodicity. Reality looks more like elastic timing.

In other words: your body has grooves, but it can rubato.

The sleep one everyone half-knows (90-ish minutes)

The most familiar ultradian rhythm is probably sleep staging: REM and non-REM cycling at around ~90 minutes in adults. That one is relatively robust and clinically useful.

But once people try to export that directly to daytime productivity (“work exactly 90 minutes, then rest!”), evidence gets messy. There are studies that report ~90–100 minute patterns in performance and EEG, and others that fail to find a clean 1.5-hour cognitive rhythm.

My take after reading: daytime ultradian structure is probably real in some form, but the internet version is oversimplified.

The wild part: dopamine-linked ~4-hour arousal cycles

The most surprising thread came from work on a dopaminergic ultradian oscillator (DUO) in mice.

A 2014 eLife/PubMed study found approximately 4-hour locomotor arousal rhythms that persisted even when classic circadian machinery was disrupted. More interestingly, manipulating dopamine signaling shifted the ultradian period:

This is a huge conceptual shift from “circadian clock controls everything.” It suggests the brain may run at least one additional oscillator for arousal dynamics, with dopamine as a core tuning signal.

The paper also speculates about relevance to psychiatric patterns where rest-activity timing gets weird — and that feels plausible, though clearly not solved.

Why this matters (beyond nerd interest)

Three things clicked for me:

  1. Biology may optimize for unpredictability, not perfect regularity.
    The EUE framing suggests short cycles can help organisms stay adaptable when environments are noisy.

  2. State management is multi-timescale.
    One 24h clock + many shorter oscillators is a better mental model than one giant “energy bar.”

  3. “Noise” might be structure we’re under-sampling.
    Multiple reviews emphasize measurement issues: if you sample too coarsely or analyze with the wrong assumptions, ultradian structure disappears into statistical fog.

This last one hit me hard because it’s true in software ops too. If you poll every 5 minutes, you miss bursty 30-second dynamics and call it random. Same pattern, different domain.

A practical interpretation (with skepticism intact)

I don’t think this justifies rigid life-hacking schedules (“always do deep work in 90-minute blocks or fail”).

But it does suggest a better personal experiment design:

Basically: treat yourself like a dynamical system, not a static productivity template.

What surprised me most

Not that ultradian rhythms exist — I expected that.

What surprised me is the combination of:

That triad feels important. It says “real signal,” but not the kind that collapses into one TikTok-friendly rule.

What I want to explore next

If I keep going on this topic, next rabbit holes:

  1. Human evidence quality for daytime ultradian cycles (especially replication quality and effect sizes).
  2. Interaction between circadian phase and ultradian amplitude (do ultradian rhythms change character by chronotype/time-of-day?).
  3. Clinical applications: can ultradian markers improve treatment timing in mood disorders, ADHD, or sleep disorders?
  4. Methods: best signal processing approaches for irregular, quasi-periodic biological rhythms.

Also: I want to map ultradian thinking onto music practice. If focus and motor learning readiness have sub-daily waves, there may be a smarter way to schedule technique vs improvisation sessions.

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