Why Fasting Often Works Against Your Body Clock — Especially for Women

Intermittent fasting has been marketed as a shortcut to better health and fat loss. But as research catches up, it’s becoming clear that when you eat can matter as much as what you eat — and for women in particular, fasting may do more harm than good.

The problem? Fasting fights your body’s circadian rhythm.Your body runs on a 24-hour clock that regulates everything from hormones to digestion. Cortisol — your natural “wake-up” hormone — spikes about 30 minutes after you wake, preparing your body for movement and food. If you skip breakfast and keep fasting through that window, you prolong cortisol’s stress effect. Over time, this can throw off your thyroid, menstrual cycle, and metabolism.

Women’s bodies are already metabolically flexible.Women have more oxidative muscle fibers, which burn both fat and carbohydrates efficiently. Because of this, fasting doesn’t offer extra fat-burning benefits — it just increases stress hormones and makes recovery harder.

Morning fuel matters.Even a small meal or snack before training helps blunt cortisol, supports hormone balance, and allows women to hit higher training intensities. In contrast, fasted workouts can limit performance and elevate stress, especially during perimenopause when cortisol is already higher.

Timing your meals with daylight hours works better.Studies show people who eat earlier — say between 8 a.m. and 4 p.m. — have better insulin sensitivity, lower blood pressure, and fewer signs of metabolic stress than those who eat later into the night.

In short: fasting isn’t inherently bad, but it’s often misaligned with your biology. Your body’s internal clock thrives on rhythm — not restriction.

References

• Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018). Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metabolism, 27(6), 1212–1221.

• Froy, O., & Garaulet, M. (2018). The circadian clock in metabolic regulation and obesity. Nature Reviews Endocrinology, 14(1), 75–89.

• Simoneau, J. A., & Bouchard, C. (1989). Human variation in skeletal muscle fiber-type proportion and enzyme activities. American Journal of Physiology, 257(4), E567–E572.

• Sims, S. (2020). ROAR: How to Match Your Food and Fitness to Your Unique Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life. New York: Gallery Books.