Women’s Core Temp Variations

The resting core temperature varies across the 24-hour circadian cycle, reaching a low shortly before waking and a high sometime in the afternoon. For women of a reproductive age, the core temperature fluctuations of the circadian cycle are superimposed on the core temperature variation throughout the menstrual cycle.

These menstrual cycle variations are seen by measuring the basal body temperature – the lowest core body temperature after a period of rest. This generally occurs in the early hours of morning, usually before waking. Core body temperature is affected in several phases of the menstrual cycles:

1. The follicular phase starts at the end of menstruation and stops with ovulation. During this phase, basal body temperature remains low.
2. At the start of ovulation, basal body temperature rises slightly.
3. During the luteal phase (post-ovulation until the start of menstruation) basal body temperature continues to rise by 0.3–0.6°C (0.5–1.0°F). It is believed this rise is caused by the thermogenic effect of progesterone.

Menstrual Cycle Influence During Exercise

Relatively little research has been done on exercise-related core temperatures in different phases of the menstrual cycle. There are conflicting results about whether or not exercise-related core temperature varies between the follicular and luteal phases. There is evidence that…

“In recreationally active females, endurance time to fatigue in hot, humid conditions and exercise tolerance time in a hot, dry environment were lower in the luteal phase compared to the follicular phase. In contrast, in well-trained women exercising in the heat, exercise performance did not differ across the menstrual cycle and high-intensity intermittent running in the heat was not influenced by the phase of the menstrual cycle”1

Understanding Individual Variations

The CORE sensor helps women to understand their own individual reactions to the different phases of their menstrual cycle. We suggest making the following observations while exercising during each phase of the cycle:

1. At a given Heat Strain Index, does exercise feel harder in one of the phases?
2. In similar environmental conditions and intensity of exercise, is a given Heat Strain Index reached quicker in one of the phases?
3. Does sweat rate and the threshold for the onset of sweating and change between the follicular and luteal phases?
4. While heat training at a given Heat Strain Index value, is heart rate decoupling from pace/power different during the follicular and luteal phases?

Additional Reading

In short, the variations in thermoregulation across phases of the menstrual cycle are not well understood by science. However, excellent information can be found in the comprehensive review Temperature regulation in women: Effects of the menstrual cycle, and also The Impact of Menstrual Cycle Phase on Athletes’ Performance: A Narrative Review.

In addition, CORE’s article ‘Women’s Menstrual Cycle Monitoring and Athletic Performance gives some easy-to-read summary insights.

References:

1. Temperature regulation in women: Effects of the menstrual cycle. Baker FC, Siboza F, Fuller A. Temperature (Austin). 2020 Mar 22;7(3):226-262.