Intermittent fasting can improve insulin sensitivity and support fat metabolism in perimenopause — but longer fasting windows raise cortisol, worsen belly fat, disrupt sleep, and risk accelerating muscle loss. The 2025 evidence shows that a 12–14 hour overnight window, combined with adequate protein and resistance training, produces real benefits. Here's the honest breakdown of what works, what backfires, and why 16:8 is the wrong default for most perimenopausal women.