Your hormones shape
every metric
you track
HRV, sleep, resting heart rate, body temperature, mood, and energy all shift predictably across your cycle. Understanding the pattern means understanding yourself - not just a single data point.
Your cycle is a monthly
performance blueprint
Each phase has a distinct hormonal signature that shapes your physiology. Your wearable captures all of it - Isabella shows you the pattern.
HRV tracks your hormones
without a blood test
Estrogen and progesterone directly modulate autonomic nervous system tone. Estrogen is parasympathetic-dominant - it raises HRV. Progesterone is sympathetic-dominant - it suppresses HRV. This creates a predictable monthly wave your wearable captures every night.
The practical implication: a "low" HRV reading in the late luteal phase is not a bad day - it is a hormonal signal. Isabella knows your cycle phase and adjusts its interpretation accordingly.
Multiple studies confirm higher HRV in the follicular phase vs luteal phase (Yildirir et al., 2002; Sato et al., 1995). The difference is driven by estrogen's direct vagotonic effect on the sinoatrial node. Oral contraceptives, which suppress the natural cycle, flatten this HRV wave - a clear marker visible in wearable data.
Your ring detects
ovulation overnight
Progesterone is thermogenic - it raises body temperature. When ovulation occurs, skin temperature rises 0.2–0.5°C and stays elevated through the entire luteal phase. This is one of the most reliable non-invasive ovulation signals available.
Wearables with continuous skin temperature sensors (such as the Oura Ring) capture this shift nightly. Ziva also lets you log daily basal body temperature (BBT) manually - combining logged entries with wearable data to confirm ovulation and estimate your luteal phase length.
Traditional basal body temperature (BBT) requires waking at the same time daily before getting up - difficult in practice. Wearables with overnight temperature sensors capture this automatically and continuously. Ziva aggregates both - wearable skin temp where available, plus manual BBT entries - for the most complete picture. The signal is the same; the method becomes far more consistent.
Your worst sleep is
predictably timed
Sleep quality fluctuates meaningfully across the cycle - and the pattern is consistent enough that Isabella can often predict your worst sleep nights from your cycle data alone.
Baker & Driver (2007) found significantly more slow-wave sleep in the follicular phase. Shechter & Boivin (2010) documented reduced sleep efficiency and more wakefulness in the late luteal phase. Progesterone metabolites (allopregnanolone) initially promote sleep but their withdrawal in the premenstrual window is a known cause of insomnia and anxiety.
Work with your cycle,
not against it
Your physiology is different in each phase. Matching training load and recovery to your cycle phase - often called cycle syncing - has strong scientific support and is one of the highest-leverage performance strategies available.
Wikström-Frisén et al. (2017) found that training loads matched to cycle phase produced significantly better strength outcomes than fixed periodisation. Estrogen has anabolic properties - follicular phase training produces more muscle protein synthesis. ACL injury risk is also phase-dependent: higher in the ovulatory window due to ligament laxity from estrogen (Zazulak et al., 2006).
PMS isn't in your head —
it's in your data
Mood, anxiety, and emotional sensitivity fluctuate significantly across the cycle - driven by estrogen and progesterone's direct effects on serotonin, GABA, and dopamine. These shifts are measurable, predictable, and show up in your wearable data before you consciously feel them.
HRV drops, sleep fragments, and resting heart rate rises in the late luteal phase - all of which Isabella correlates with the mood tags you log. Over time, the pattern becomes undeniable: your data knows your cycle before you do.
Estrogen increases serotonin transporter expression and reduces MAO-A activity (Bethea et al., 2002). Allopregnanolone withdrawal triggers anxiety via GABA-A receptor changes - the same mechanism as benzodiazepine withdrawal (Backstrom et al., 2014). HRV is lower during symptomatic PMS days, with the drop correlating with symptom severity (Bai et al., 2009).
Chronic stress can
silence your cycle
The HPA axis (stress) and HPG axis (reproductive hormones) are directly connected. High cortisol suppresses GnRH - the signal that drives the entire hormonal cascade of ovulation. Isabella watches for this pattern in your data.
When the cycle isn't typical
Ziva knows your health context - set up at onboarding or by your provider. Isabella uses this to interpret your data differently and flag relevant patterns for each person.
Every phase. Every signal.
Every day.
At setup, you tell Ziva about your cycle. Your wearable captures what happens every night. Isabella connects both streams - building a personal cycle model that improves with every month of data.
The blood tests that
complete the picture
Wearable data shows what your hormones are doing to your body. Lab tests show what your hormones actually are. Together, they tell the full story.
Common questions, answered
Frequently asked
Your cycle is data.
Isabella reads it
for you, every day.
HRV, skin temperature, sleep, symptoms, and cycle phase - all connected into one personalised picture of your hormonal health.