Fuel, data and moving through symptoms

Fuel: the best-supported part

• The International Olympic Committee's 2023 consensus says low energy availability, meaning not eating enough for the amount you exercise, is linked to harm to health and performance in both women and men. The problem is called REDs.¹
• Stanford researchers say a missing period can be a warning sign of underfueling, which can affect bone health. In a seven-year program, runners worked with dietitians on eating enough for their training, and bone stress injuries in the most fuel-sensitive bones dropped nearly in half.²
• Dr. Sims also stresses fueling your workouts every day, with carbohydrates for high-intensity sessions and protein spread across meals.³

What's less clear is whether you should change how you eat by phase. A review says it isn't entirely clear how food intake varies across the cycle or whether that changes performance.⁴ On average, people eat about 168 more calories a day in the luteal phase.⁵

Using a watch or ring

Resting heart rate and heart rate variability shift with sleep and with your cycle. Stanford researchers analyzed wrist-tracker data from over 42,000 cycles. Losing about 5 hours of sleep across a week lowered HRV by nearly 3%, whatever the phase. A single reading can't tell you whether a low score comes from poor sleep or from where you are in your cycle.⁶

Women who slept less than about 7.3 hours a night tended to have more irregular cycles.⁶ The people in that study used WHOOP trackers, who tend to be more active and wealthier than average, and the study left out people on hormonal birth control and those who were pregnant or perimenopausal. It may not describe everyone.⁶

Myth: Cycle syncing your diet, work and social life is proven.

Cycle syncing is everywhere online, and it caught on for an understandable reason: it gives you permission to listen to your body. Cleveland Clinic notes the idea came from a 2014 book and that the cycle-syncing program hasn't been researched in a clinical setting.⁷ So it's popular, but unproven. Paying attention to your own energy and mood is useful. Treating a phase-by-phase plan as a rule isn't supported by research yet.

Exercise can ease period symptoms

A 2024 analysis of 49 trials with 3,129 people found that every type of exercise tested reduced period pain, with clear improvement after 4 to 8 weeks of regular training.⁸ A 2025 analysis of aerobic exercise found similar results, though the studies differed a lot from each other.⁹ ACOG says regular aerobic exercise lessens PMS symptoms for many women, and that it works best when you do it regularly, not only on days with symptoms.¹⁰

What to do with this

• Notice your own pattern for a few cycles: energy, sleep, mood and how workouts feel.
• Lift consistently, and ease off on days that feel rough.
• Eat enough to cover your training, every day.
• Talk to a doctor if your periods stop or change while you're training hard or eating less.