Real-Time Stress Explained

What is Real-Time Stress?

Real-Time Stress uses HRV, or heart rate variability, to reflect the body's current physiological load:

• HRV measures changes in heartbeat intervals and reflects the balance of the autonomic nervous system, including sympathetic and parasympathetic activity, as well as recovery capacity.
• HRV itself is not stress, but it can be used as an objective indicator of stress-related physiological load.

HRV and Real-Time Stress move in opposite directions:

• HRV below your personal baseline -> sympathetic activity is more dominant, and body load or stress may be higher.
• HRV above your personal baseline, or at a relatively high level -> parasympathetic activity is more dominant, and the body is in a more relaxed and recovered state, with lower stress.

Note: HRV is affected by sleep, exercise, cycle hormones, environment, mental state, and many other factors. A single value cannot independently determine stress. It should be interpreted together with your personal baseline and trend analysis.

How stress affects the menstrual cycle

Existing evidence suggests that long-term or repeated physical and psychological stress can affect menstrual regularity through neuroendocrine mechanisms:

• Physical or psychological stress activates the hypothalamic-pituitary-adrenal axis (HPA axis), which may suppress the hypothalamic-pituitary-ovarian axis (HPO axis), leading to delayed ovulation or cycle disruption.
• Sympathetic activation caused by stress is often accompanied by lower HRV, which is consistent with increased physiological load.
• The impact of stress on cycle regulation is also supported by large-scale wearable data, where stress is associated with variability in menstrual cycle length (Ultrahuman).

How the menstrual cycle affects HRV and stress

Research shows that hormone changes across menstrual cycle phases can affect HRV:

• Higher progesterone during the luteal phase is associated with lower HRV, reflecting lower vagal activity.
• Hormone changes in the follicular phase and other stages may also affect HRV, although findings differ across studies (PubMed).

These are cyclical physiological fluctuations, not a single stress response. A cyclical HRV drop should not automatically be interpreted as stress itself increasing.

Why women should pay attention to HRV and stress metrics

1. Distinguish the main driver

It helps you judge whether a current HRV drop is driven by normal cyclical hormone change or by additional physical stress load.

2. Reduce misunderstanding and anxiety

It helps users understand that lower HRV during some phases can be part of physiological rhythm, not necessarily an abnormal stress state.

3. Better body awareness and control

Combining cycle phase and stress trends can support more precise rest, recovery, and behavior choices.

References

1. S. M. El-Heriti et al. Changes in resting heart rate variability across the menstrual cycle. Psychophysiology. 2014;51(8):775-782. DOI:10.1111/psyp.12219 (PubMed)
2. S. M. De Zambotti, J. Trinder, A. Silvani, et al. Menstrual Cycle Changes in Vagally-Mediated Heart Rate Variability are Associated with Progesterone. Journal of Clinical Medicine. 2020;9(3):617. DOI:10.3390/jcm9030617 (PubMed)
3. Ved Asudani, Prakhar Chauhan, et al. Impact of BMI, stress, and activity on menstrual cycle length and variability: insights from 4055 cycles. Ultrahuman Ring AIR Study, 2025. (Ultrahuman)
4. E. Rosano, G. J. Connell. Heart rate variability in women during a mental stressor: the effect of the menstrual cycle and post-menopause. The Physiological Society, 2025. (physoc.org)