Why Active Women Should Break Their Fast at Breakfast (Not Noon)

Sims frames this protocol as the natural human fasting rhythm — the one our bodies are actually adapted for. By finishing food intake after dinner and not eating again until breakfast, the body completes digestion before sleep onset, allowing the parasympathetic nervous system to dominate overnight. This enables restorative sleep architecture, proper overnight blood glucose regulation, and the natural circadian hormonal cascade. In contrast, when fasting is extended into the late morning or afternoon, the circadian rhythm is disrupted, appetite hormones like ghrelin surge, spontaneous physical activity drops due to low energy, and calories get compressed into a late-day window — forcing the body to digest during the sleep phase. This overnight fast protocol is presented as the healthy middle ground that gives women the benefits of fasting without the costs.

Sims positions circadian-aligned fueling as the foundational principle that should guide all eating patterns for women. The modern trend of delaying the first meal and loading calories at the end of the day is, in her view, a direct driver of the low energy availability problem. When women skip breakfast, they experience reduced incidental movement throughout the morning, hit an energy wall in the afternoon, and then become ravenously hungry at night — precisely when the digestive system should be winding down. This pattern also disrupts sleep quality, creating a feedback loop of fatigue and hormonal dysregulation. Her solution is simple but countercultural: eat breakfast, eat during the day when the body is active and metabolically primed for intake, and stop eating after the evening meal. This realigns behavior with biology.

Sims draws a direct line from the current diet culture — intermittent fasting, calorie restriction, 'eat less, move more' — to the rising rates of infertility and anovulatory cycles. She explains that many women experiencing infertility are still having monthly bleeds, which creates the false impression that they are ovulating. In reality, these are anovulatory bleeds where estrogen builds the endometrial lining but ovulation never occurs, so there is no progesterone opposition and no viable egg. The bleeding is just the shedding of unopposed endometrial tissue. When clinicians and patients look at fueling — increasing energy availability, reducing fasting windows, reducing systemic inflammation — ovulation often resumes. This protocol is as much for self-assessment as for clinical practice.

Sims explains that when women extend their fast beyond the natural overnight window, the consequences cascade through multiple systems. Bookending calories to the end of the day means the body is still digesting during sleep onset, preventing the parasympathetic activation necessary for restorative sleep. This disrupts circadian rhythm, appetite hormones, and reduces incidental movement during the day due to fatigue — people hit an 'afternoon wall.' The result is a vicious cycle: fasting all day leads to overconsumption at night, poor sleep, and next-day exhaustion. She emphasizes that a 12–13 hour overnight fast — stopping after dinner and eating breakfast — actually helps sleep, circadian rhythm, parasympathetic tone, body composition, and blood glucose regulation because the body can enter solid sleep without active digestion. This is the natural human rhythm she argues we should follow, not the prolonged daily fasts promoted in popular media.

Sims describes a clinical blind spot: active women walk into their doctor's office with a ferritin of 50 ng/mL — a level that five years ago would have triggered intervention and supplementation protocols — and are now told '50, wow, that's amazing.' The threshold for concern has drifted down to 20 because the population mean has been dragged lower by widespread sedentary behavior and poor metabolic health. This means active women with genuinely inadequate iron status are being reassured and sent home. The same dynamic plays out across other markers like blood lipids. Sims frames this as a statistical problem rooted in the bell curve: when the population shifts unhealthier, the 'normal' range expands downward, and the active minority becomes invisible within it. The consequence is that over half of recreationally active women are unknowingly underfueling, accumulating subtle but real physiological costs that never reach the threshold for clinical intervention.

Sims counters this narrative by explaining what is actually happening physiologically during a multi-day fast in women. The 'better sleep' and feelings of improvement that fasters report after day 3 or 4 are not signs of enhanced health — they are signs that the body is conserving energy by downregulating systems. Thyroid function takes a measurable hit within 72 hours of low energy availability, which cascades into reduced metabolic rate, lower body temperature, and decreased energy expenditure. Lean mass is catabolized for fuel. The entire endocrine system dials back. Sims is particularly critical of male-led fasting advice ('bro science'), noting that men can tolerate these protocols somewhat longer because they don't have the additional energy demands of a menstrual cycle and the greater endocrine load that female reproductive function requires. For women, the cost of extended fasting is paid in endocrine disruption that can persist well beyond the fast itself.

Sims responds to this argument by pointing to what actually happened to populations during periods of food scarcity. Famine times were not periods of thriving — they were periods of reproductive shutdown. Women's bodies, when faced with insufficient energy, downregulate endocrine function, cease ovulation (or produce anovulatory cycles where bleeding occurs without egg release), increase fat storage as a protective mechanism, and become effectively infertile. Population booms didn't occur during lean times; they occurred during feast periods when food was abundant enough to sustain female reproductive function. Tribes that had greater food availability survived and reproduced; those that didn't either died out or migrated. She ties this directly to modern fertility issues — when clinicians see unexplained infertility, the first investigation should be into fueling patterns, inflammation, and diet trends, because correcting energy availability often restores fertility. The ancestral precedent, she argues, actually supports the opposite conclusion from what fasting proponents claim: food abundance, not scarcity, enables female health and species continuation.