Cycle Tracking & Cycle-Based Training

What happens in the cycle

Picture two waves. In the first half, the follicular phase, estrogen rises after menstruation to a peak around ovulation. In the second half, the luteal phase, progesterone takes over, estrogen makes a smaller second wave, and without pregnancy both drop, the signal for the next period.

These hormones are not purely reproductive hormones. Estrogen acts on muscles, bones, blood vessels, the brain and mood. Progesterone tends to be calming and sleep-promoting, and it raises body temperature. That is why the temperature method works, and that is why wearable users see their luteal phase in the data.

What wearables show

In the luteal phase, core temperature rises, resting heart rate climbs by a few beats and HRV falls. A systematic review of 16 wearable studies found HRV higher at the start of the cycle and lower toward the end, with differences of 3 to 9 percent in the time-domain measures. That is physiology, not a stress slump. Anyone who knows their HRV baseline reads it better in relation to the phase; otherwise you read a slump into it every month that is simply progesterone.

How accurate the temperature is depends heavily on where it is measured. In a comparative study with 17 female athletes, a continuous sensor inside the body showed the largest rise from the follicular to the luteal phase, 0.31 degrees on average, and detected all ovulations. Measured under the tongue, it was only 0.17 degrees. A network meta-analysis of 27 studies arrived at a pooled accuracy of 0.88 for wearables in detecting the fertile window, best with rings and multiple measured parameters.

Metabolism, iron and sleep

Energy expenditure rises slightly in the luteal phase. A systematic review of 7 recent studies found a trend toward higher resting metabolic rate in 5 of them, estimated at 30 to 120 kcal per day, or about 3 to 5 percent. That lies within the range of normal daily fluctuations and measurement error. Many women have more appetite before their period, and those who know this plan protein and complex carbohydrates deliberately instead of being surprised by cravings.

Iron is lost with every menstruation, and iron deficiency is among the most common nutrient deficiencies in women of reproductive age, with fatigue, a drop in performance and feeling cold. Ferritin therefore belongs in every blood test. Around the period and in the late luteal phase, many women report poorer sleep as progesterone falls.

The data gap

Many studies cited in biohacking circles were conducted mostly on men. In the US, the inclusion of women in clinical trials only became a legal requirement in 1993, and in sports science, studies that determine the cycle properly are still a minority today. The cycle was long regarded as a confounding variable, so researchers preferred to study men. With every recommendation it is worth asking whether the data include women at all. This also applies to fasting and cold: aggressive fasting protocols hit the stress axis and the cycle harder in some women, and an absent cycle is never an optimization success but a warning sign of too little energy.

What the studies show

McNulty 2020 — performance across the cycle phases

Systematic review and meta-analysis of 78 studies with naturally cycling women, 51 of them in the pairwise analysis. Compared with all other phases, performance in the early follicular phase was only trivially lower, with a pooled effect size of minus 0.06. Study quality was rated low overall. The authors therefore do not recommend general rules but an individual approach.

Niering 2024 — maximal strength

Meta-analysis of 22 studies with 433 female participants. For isometric maximal strength it found a moderate advantage of the late follicular phase over the early one, with an effect size of 0.60; for isokinetic strength a small advantage around ovulation; for dynamic strength only 0.14. The largest effect rests on 7 studies with a heterogeneity of 95 percent. This paper thus contradicts the other meta-analyses, but stands on shaky ground.

Colenso-Semple 2025 — muscle building

Randomized crossover study with 12 young women, each doing 6 days of strength training in the late follicular and the mid-luteal phase, with cycle phases confirmed by blood and urine tests. Muscle protein synthesis rose markedly with training but did not differ between the phases, and neither did protein breakdown. Muscle is not more anabolic in any phase.

Hurtová 2026 — energy expenditure

Systematic review of 7 studies since 2020 on resting metabolic rate across the cycle. 5 studies showed a trend toward higher expenditure in the luteal phase, 4 of them significant, at an estimated 30 to 120 kcal per day. The authors point out that the effect is small and may overlap with normal daily fluctuations.

Where the data stop

Cycle-based training plans by template are not supported: heavy weights in the first half, easy in the second. The meta-analyses mostly find trivial differences; a fourth finds moderate ones, but with very heterogeneous studies. Many papers did not confirm the cycle phase hormonally but only estimated it by calendar. Some women feel clear differences, others hardly any, and that is exactly why your own pattern beats any generic template.

The popular claim that basal metabolic rate rises by 100 to 300 kcal before the period does not hold up against the current review. For seed cycling and cycle supplement bundles there is practically no reliable evidence. And the accuracy of wearables applies to the fertile window in studies, not as a method of contraception.

Status, approval and legal

Cycle tracking is not a medical treatment but self-observation, with a wearable, an app or a notebook. Only products specifically approved for this purpose are intended as a method of contraception. Cycle data reveal a lot, from pregnancy to menopause. Before downloading an app, it is therefore worth looking at the privacy policy: where are the data stored, are they shared, is there an anonymous mode? Apps with local storage and European data protection are preferable.

Safety

Observing is harmless; the risks lie in misinterpretation. Under hormonal contraception the natural hormone waves are largely switched off, cycle-based training makes no sense there, and HRV patterns also look different; in pill users HRV is lower on average. An absent or very irregular cycle, very severe pain or bleeding and severe premenstrual mood slumps should be investigated by a physician. The severe form, premenstrual dysphoric disorder, is a recognized diagnosis and not a self-optimization project. Magnesium and vitamin B6 are discussed for PMS. Questions about contraception, the wish to have children and menopause belong in a gynecologist’s practice.

Sources

• McNulty et al., Sports Med 2020 (meta-analysis, performance)
• Niering et al., Sports (Basel) 2024 (meta-analysis, maximal strength)
• Colenso-Semple et al., J Physiol 2025 (muscle protein synthesis)
• Hurtová et al., Front Physiol 2026 (resting metabolic rate)
• de Jager et al., Sports Med 2026 (HRV and wearables)
• Shi et al., NPJ Digit Med 2026 (wearables, fertile window)
• Nolte et al., Front Sports Act Living 2025 (temperature measurement)
• Li et al., BMC Womens Health 2026 (exercise for PMS)
• D Souza et al., J Appl Physiol 2023 (cycle, pill and physiology)