Dr. Stacy Sims: “Your Wearable Is Lying To You!” How To Train With Your Cycle

Sims argues that the lack of robust, individualised studies means blanket cycle-phase training advice is often too complex and not useful for most women. Instead, she proposes a self-experimentation approach: each woman should track her own data (cycle length, sleep, perceived energy) and then backwards-engineer her best and worst days. On high-energy days, you want to impose a significant training stress because your body is able to handle and supercompensate from it — this is the core mechanism of getting fitter. On days when you feel flat, trying to push hard can tip you into an under-recovered state, which compounds with normal life stress and can degrade resilience across the board. The protocol is as much about life management (hard meetings, work from home) as about exercise.

Sims details that after ovulation, progesterone raises resting heart rate and respiratory rate; the wearable reads this as a drop in HRV and labels you unrecovered, but the effect is isolated to one vagal nerve — muscles and brain may be fully ready. The 4–5 days before menstruation are particularly problematic: hormonal flux and inflammation prevent a green score regardless of actual recovery. She therefore instructs women to step back and ask 'How do I really feel?' rather than letting the device dictate their day. By plotting HRV across an entire month, you can see whether your baseline is improving (adaptation) or declining (overtraining) — a far more valuable signal than the daily colour.

Sims shared that she has had enough conversations with athletes who saw a red recovery score and then mentally checked out before they even started, leading to poorer outcomes. Because there is no day in the cycle when a woman cannot perform, the wearable's red flag is misleading. By removing the device, the athlete is forced to rely on internal cues and preparation, which is both more accurate and less damaging. This advice runs counter to the quantified-self movement and is a strong example of 'data is not always helpful'.

Sims acknowledged that if you have the technology to know exactly when you've ovulated, you can tap into molecular-level insights: progesterone's arrival signals a metabolic shift that reduces glycogen efficiency and increases protein turnover, meaning you need more fuel and building blocks. Similarly, because high-intensity work is more glycolytic and produces more metabolic stress, leaning into steady-state can prevent overreaching. However, she quickly pivots to stressing that this level of granularity is not necessary for most women and that individual pattern tracking is far more accessible and robust.

Sims explains that women exhibit a different intrinsic beat-to-beat rhythm that gets flagged as an anomaly. More critically, after ovulation, rising progesterone increases resting heart rate and respiratory rate, which the algorithms read as a drop in HRV and hence a red recovery score. But according to Sims, this vagal effect is isolated — muscles and brain may be fully recovered. In the 4–5 days before menstruation, when progesterone plummets and inflammation rises, the algorithms almost never allow a green score, yet performance capacity is not compromised. She says the psychological impact of seeing red can become a self-fulfilling prophecy, especially before competition. Therefore, she advises women to treat wearables as a trend tool over the full cycle, not as a daily verdict.

Sims notes that day 1 is the first day of bleeding; cycle length can vary and is normal up to a few days fluctuation. She recommends women track how they sleep and feel across the cycle, because after a few months patterns emerge. For example, many women feel fantastic around day 12 and flat around day 21. On the fantastic days, she says to book hard meetings and train with high intensity to get a training stress that the body can absorb and recover from, driving fitness. On flat days, the advice is to work from home, spend time in nature, and prioritize parasympathetic recovery to avoid pushing into an overreached or under-recovered state, which can then harm overall stress resilience. The key insight: go hard when you are stress-resilient, back off when you aren't, to prevent compounding life stress and under-recovery.