Perimenopause can start as a loss of rhythm, not a missed period

There is a way of talking about perimenopause that arrives late.

We wait for the period to become clearly irregular. We wait for hot flashes to be obvious. We wait for the woman to say: "something changed," but months late.

Circadian rhythm science suggests another possibility: before the menstrual calendar becomes visibly disordered, the contrast between day and night may begin to fade.

It is not just sleeping too little. It is sleeping without recovering. Waking up hot. Feeling that the night did not turn the body off. Having fatigue in the morning even when the clock says you slept enough. Noticing that coffee, a late dinner, nighttime light, or a heavy shift costs you more than it used to.

That does not prove perimenopause. But it can be a system signal: your biological clock is losing amplitude.

The body does not just keep time. It has contrast.

A healthy circadian rhythm is not just "sleeping at night and waking during the day." It is a contrast system:

• light and activity during the day;
• darkness and rest at night;
• high cortisol in the morning and lower cortisol when you go to bed;
• temperature, heart rate, and autonomic recovery moving in a pattern;
• food and caffeine kept away from the biological night.

When that contrast flattens, a person may sleep many hours and still live through a weak night. The body was in bed, but not necessarily in repair mode.

In women, that amplitude sits on top of a second oscillation: the hormonal cycle or reproductive transition. Estradiol, progesterone, temperature, sleep, vasomotor load, and the HPA axis do not move separately. They interfere with each other, compensate for each other, or amplify each other.

That is why the same misalignment does not cost the same at 28 as at 47. And it does not cost the same in the follicular phase as in the luteal phase.

What changes in the transition

Perimenopause is not a hormonal blackout. It is variability.

There can be ovulatory and anovulatory cycles interspersed. High estradiol one week and low the next. Intermittent progesterone. Erratic FSH. More fragile sleep. More awakenings. More thermal sensitivity. More of the feeling that "my body no longer responds the same way."

The Lua Labs hypothesis is that part of that transition may be seen as a loss of circadian amplitude: day and night stop being so separate.

A night with many awakenings does not just feel bad. It also alters endocrine signals. In an experimental model of menopause published in The Journal of Clinical Endocrinology & Metabolism, sleep fragmentation increased cortisol at bedtime and reduced the cortisol awakening response. In other words: it was not just a "bad night." It was an alteration of the HPA axis.

Another study in women in transition showed that more awakenings were associated with reproductive hormone profiles closer to postmenopause, even after controlling for vasomotor symptoms and depression.

The pattern starts to take shape: broken sleep, unstable thermoregulation, and misaligned cortisol can form a nighttime signature of transition.

Nighttime temperature matters

Many women describe perimenopause as heat. But "heat" is a poor word for a complex system.

There can be hot flashes. There can be sweating. There can be micro-awakenings. There can be a peripheral temperature that no longer drops or rises with the same pattern. There can be nights when the body cannot find a comfortable thermal zone.

Temperature is a circadian signal. In studies with actigraphy and distal temperature, greater amplitude and stability of the temperature rhythm have been associated with better sleep efficiency, less time awake after falling asleep, and more total sleep.

That does not mean a wearable can "measure your hormones" directly. It cannot. Wrist or finger temperature is not estradiol, progesterone, or cortisol. But it can provide a rhythm clue when interpreted against your own baseline.

The key is not to compare women with one another. The useful question is not "is your temperature high or low?" The question is: is your night losing its usual pattern?

Sleeping 7 hours does not mean your night repaired you

This is the part that gets underestimated most.

Many women arrive at a visit with a similar phrase: "I sleep, but I wake up tired." From the outside, it looks like anxiety, stress, or age. From the inside, it feels like living with a battery that never fully charges.

In perimenopause, a night can add up to 7 hours and still include:

• awakenings from heat;
• a heart rate higher than usual;
• lower HRV;
• a feeling of alertness when you go to bed;
• nighttime hunger or cravings;
• worse energy the next day.

Sleep duration is a necessary metric, but an insufficient one. Circadian amplitude asks something more precise: did your body clearly distinguish night from day?

The role of food, light, and shifts

The central clock responds strongly to light. The peripheral clocks — liver, gut, pancreas, adipose tissue — respond strongly to food, caffeine, activity, and timing.

If the night brings bright light, a late dinner, a snack, alcohol, caffeine, stress, or a night shift, the body receives contradictory signals. The brain tries to sleep. Metabolism tries to process. The HPA axis tries to sustain alertness. Thermoregulation tries to stabilize.

In a 30-year-old woman with high circadian resilience, that collision may feel like one bad night. In a 47-year-old woman with intermittent progesterone, hot flashes, and fragile sleep, the same collision can feel like three days of fatigue.

It is not moral. It is not "you stayed up late and that is why you feel bad." It is temporal biology.

That is why Lua Labs has been building scores such as OCCS, LMPS, MMCI, NML, and SWCD: ways of thinking about the coherence between ovary, sleep, food, light, and work. The next conceptual step is to integrate it into something broader: a circadian amplitude score.

CACIS: a digital biomarker hypothesis

Lua Labs proposes an experimental biomarker called CACIS: Circadian Amplitude Coherence Integrative Score.

It is not diagnostic. It is not a clinical promise. It is a product and research hypothesis: a way to look at whether a person's day-night signal preserves enough contrast.

A future CACIS would have to consider:

• sleep regularity and continuity;
• nighttime temperature against the personal baseline;
• recovery proxies such as HRV and resting heart rate;
• daytime activity and nighttime rest;
• zeitgeber load: nighttime light, late food, caffeine, shifts;
• cycle phase or hormonal stage.

The output should not be "you are in perimenopause." That would be a bad reading.

The right output would be closer to: "this week your night-day signal lost contrast; the factors that contributed most were late dinner, heat-related awakenings, and irregular sleep."

That is actionable. And above all, more honest.

What this changes for a woman

Perimenopause is usually told as a list of symptoms: hot flashes, insomnia, irritability, fatigue, cycle changes.

But a list does not explain why everything appears together.

The circadian framework makes it possible to see something else: maybe these are not isolated symptoms, but a loss of coherence. The body no longer separates day from night as well. The night no longer repairs in the same way. The morning no longer starts in the same way. Late food weighs more. Nighttime light weighs more. Stress stays switched on longer.

For Carmen, 47, that matters because her doctor may say "it is age." But she does not need resignation. She needs context. She needs to see patterns. She needs to be able to say: "my worst days come after nights with heat, late dinner, and high heart rate, even if I slept 7 hours."

That changes the conversation.

Be careful with blame

There is a trap in all of this: turning circadian rhythm into another individual responsibility.

"Eat earlier." "See the sun." "Sleep better." "Turn off screens."

That can be useful for some women. But it can also be unfair. Many women work nights, care for children or parents, live with noise, heat, insecurity, or schedules they do not control.

The right reading is not to blame. It is to measure context.

If a woman cannot regulate her schedule because of work or caregiving, the product should not tell her she failed. It should help her understand which part of the context weighs more and which pattern repeats in her body.

Longitudinal hormonal intelligence does not begin with a perfect recommendation. It begins by stopping treating the female body as if it were the same every day.

This article summarizes peer-reviewed scientific evidence for educational purposes. It does not replace medical evaluation, diagnosis, or treatment. Lua does not diagnose perimenopause or interpret clinical results.

Selected sources

• Ren X, Wang W, Li W, et al. Circadian rest-activity rhythms and multimorbidity and mortality risks among menopausal women: a trajectory analysis of a UK Biobank cohort. BMC Public Health. 2025. DOI: 10.1186/s12889-025-22536-3.
• Hou SY, Chiu CJ, Shea JL, et al. Role of age, menopausal status, and symptoms in midlife women: examination of sleep patterns and rest-activity circadian rhythms. Sleep Medicine. 2024. DOI: 10.1016/j.sleep.2023.11.015.
• Cohn AY, Grant LK, Nathan MD, et al. Effects of Sleep Fragmentation and Estradiol Decline on Cortisol in a Human Experimental Model of Menopause. Journal of Clinical Endocrinology & Metabolism. 2023. DOI: 10.1210/clinem/dgad285.
• Coborn J, de Wit A, Crawford S, et al. Disruption of Sleep Continuity During the Perimenopause: Associations with Female Reproductive Hormone Profiles. Journal of Clinical Endocrinology & Metabolism. 2022. DOI: 10.1210/clinem/dgac447.
• Tai Y, Obayashi K, Yamagami Y, Saeki K. Association between circadian skin temperature rhythms and actigraphic sleep measures in real-life settings. Journal of Clinical Sleep Medicine. 2023. DOI: 10.5664/jcsm.10590.