Dr. Margit Zeitler is a chronobiologist and professor of integrative physiology at Arizona State University, where she directs the Temporal Ecology and Human Rhythms Lab. She arrived in Phoenix in 2019 to study thermoregulatory behavior in desert reptiles and, within three years, had pivoted entirely to human subjects, a shift she describes, with characteristic understatement, as "following the more alarming data." For the past six years, she has been building one of the largest longitudinal datasets on climate-driven circadian disruption in a heat-exposed urban population, tracking sleep architecture, hormonal rhythmicity, and mood cycling in a cohort of 11,000 Maricopa County residents. She also wears a research-grade actigraph on her own wrist and has not missed a night of personal sleep data collection since 2021. We spoke in her office, where the blinds were drawn against a March afternoon that had already reached 98°F.¹

Note: This interview is a work of speculative journalism. Dr. Zeitler is a fictional composite, but the science she discusses is real and sourced throughout.

You started with lizards.

Zeitler: I started with lizards. Sonoran Desert species, specifically. I was studying how they modulate their activity windows as thermal environments shift. When they hunt, when they rest, how the timing of their entire behavioral repertoire reorganizes around temperature thresholds. Beautiful, elegant work. Very fundable. And then it was the summer of 2022, Phoenix hit forty-something consecutive days above 110, and I was sitting in my apartment at midnight. Still 99 degrees outside. And I thought: I am the lizard. I am reorganizing my behavioral repertoire around temperature thresholds. I'm grocery shopping at 5 AM. I'm running at 4:30 in the dark like a criminal. And I realized nobody was building the dataset for what this does to human biological timing at the population level.

When you say "biological timing," you mean something more specific than sleep schedules.

Zeitler: Much more specific. Temperature is a zeitgeber, a time-giver. One of the primary environmental signals your body uses to locate itself in the 24-hour cycle. The mechanism is precise: as you approach sleep, your body dilates blood vessels in the hands and feet to dump core heat. That temperature drop is the onset signal. It tells your brain: now. When ambient temperature stays elevated through the night, that signal degrades. Not catastrophically. You still fall asleep, eventually. But the architecture changes. You get less slow-wave sleep. And deep sleep is when the brain clears amyloid proteins, consolidates immune function, pulses growth hormone and reproductive hormones.²

So the clock isn't broken. It's receiving a corrupted input. And the body, being adaptive, adjusts to the corrupted input. Which is actually worse than if it just broke.

Why worse?

Zeitler: Because you don't notice. A broken clock, you'd feel terrible, you'd seek help. A clock that recalibrates slowly, over years, to a degraded signal? You just become someone who sleeps a little less deeply. You attribute it to age, or stress, or your phone. The Fudan study last year, 23 million sleep-nights, found a 20% increase in sleep insufficiency for every 10°C rise in ambient temperature.³ Under high-emission scenarios, that's 33 hours of lost sleep per person per year by 2100. But those are averages. In a city like Phoenix, we're already seeing numbers that exceed the end-of-century projections for cooler regions.

Your cohort data shows this.

Zeitler: Our cohort shows a measurable decline in slow-wave sleep duration that correlates with nighttime urban heat island intensity, even after controlling for age, AC access, and socioeconomic variables. The effect is strongest in women, whose thermoregulatory cycling begins earlier in the evening, making them more sensitive to elevated ambient temperatures at the critical pre-sleep window, and in adults over 60.⁴

But the finding that unsettles me most has nothing to do with magnitude. It's the invisibility. We have participants whose deep sleep has declined 15% over four years who report, on questionnaires, that their sleep quality is "about the same."

The body converts.

Zeitler: Exactly. The subjective experience normalizes faster than the physiology deteriorates. You stop noticing what you've lost because the loss is incremental and the body is, frankly, very good at making do. We are spectacular at making do. It's our worst quality.

I want to ask about the mood work. The SAD findings.

Zeitler: [leans back] This is where I become speculative, and I want to be honest about that. The established science is clear: seasonal affective disorder tracks latitude. Higher latitude, less winter light, more SAD. Prevalence of 9.5% in northern Finland, nearly 9% in Alaska, subsyndromal rates approaching 44%.⁵ The mechanism runs through melatonin and serotonin pathways entrained to photoperiod. Fine. Well-understood.

But what we evolved is a compound signal. Light and temperature together tell the body what season it is. Climate change is decoupling those signals. Winters are warming faster than photoperiod is changing, obviously, because photoperiod doesn't change. The Earth still tilts. So you get novel combinations: December light with October temperatures. And we're starting to see, and this is preliminary, mood-cycling patterns in our mid-latitude cohorts that resemble what we'd historically associate with higher latitudes.⁶

Seasonal depression migrating south.

Zeitler: Or, and this is the part that really keeps me up — no pun intended — summer-pattern SAD migrating north. Summer-pattern SAD is the less-studied variant where depressive symptoms onset with heat and remit with cooling. Historically a lower-latitude phenomenon. As extreme heat pushes into regions that never experienced it, we may be watching the entire affective geography of the continent reorganize. But the surveillance infrastructure to see it clearly doesn't exist yet. We're inferring it from residuals in datasets that weren't designed to capture it. Which is a polite way of saying we're squinting.

What about reproductive timing? There's a theoretical pathway from circadian disruption to fertility cycles.

Zeitler: There is, and it's robust mechanistically. Melatonin modulates the HPG axis, sleep architecture governs hormonal pulsatility, the whole reproductive timing system is downstream of circadian integrity.⁷ But I have to be honest: we don't have the population-level data yet. The tools to measure drift in ovulatory timing or LH surge patterns at the scale of climate are only now being built.

I find this more unsettling than a clean finding, actually. The mechanism says the effect should be there. We just can't see it yet. We're in the period before the measurement catches up to the biology. That gap is not reassuring.

You track your own sleep data.

Zeitler: [pause] I do. Ten years of continuous actigraphy. I have watched my own slow-wave sleep decline in a pattern that is consistent with, but not exclusively attributable to, the thermal environment I live in. I'm also aging. I'm also stressed. I'm also a woman in her fifties living in the hottest large city in America. I cannot disaggregate the contributions.

[She gestures at her wrist.]

That inability to separate the signal from the noise in my own body, while knowing exactly what the signal looks like in a population of 11,000. I keep reaching for a word for it. It's not anxiety. The closest I've come is: being a cartographer who can see the map shifting but can't locate herself on it.

Do you sleep well?

Zeitler: [long pause] I sleep. I don't know what "well" means anymore. I'm not sure it's a category that survives what I know.

Last question. What do you tell people who ask what to do?

Zeitler: I tell them to look at their data. Get a wearable, track their deep sleep, watch it over years. Not because the information will fix anything. But the body is already keeping score. You might as well read the scorecard.

And then I tell them that air conditioning is a deferral. It keeps the room cool while the signal outside continues to corrupt. Every individual who sleeps well in a cooled room is one more person whose body doesn't register what's happening to the population.

Footnotes

1. Phoenix recorded its earliest calendar-year date reaching 98°F in March 2025. Current projections suggest this threshold will continue advancing. The 2026 date used here is extrapolated. ↩

2. Meyer, N. et al. "Molecular basis of complex relationships between climate change, sleep disorders, and Alzheimer's disease." Egyptian Journal of Neurology, Psychiatry and Neurosurgery 61, 27 (2025). https://ejnpn.springeropen.com/articles/10.1186/s41983-025-00957-6 ↩

3. Li, A. et al. "Climate warming may undermine sleep duration and quality in repeated-measure study of 23 million records." Nature Communications 16, 2609 (2025). https://www.nature.com/articles/s41467-025-57781-y ↩

4. Fisher, L. "Losing Sleep Over Climate Change: The Hidden Impact of Rising Temperatures." Yale Global Health Review (2025). https://yaleglobalhealthreview.com/2025/05/18/losing-sleep-over-climate-change-the-hidden-impact-of-rising-temperatures/ ↩

5. Kim, K. et al. "Global prevalence of seasonal affective disorder by latitude: A systematic review and meta-analysis." Journal of Affective Disorders (2025). https://www.sciencedirect.com/science/article/abs/pii/S0165032725012492 ↩

6. Quera Salva, M.A. et al. "Seasonality of brain function: role in psychiatric disorders." Translational Psychiatry 13, 76 (2023). https://www.nature.com/articles/s41398-023-02365-x ↩

7. Gonzalez-Aleman, G. et al. "Ecological and evolutionary consequences of changing seasonality." Science 388, eads4880 (2025). https://www.science.org/doi/10.1126/science.ads4880 ↩