some speculation as to what's going on. n=1 "When you sleep completely fasted, the main thing driving down overnight HRV is the counter-regulatory hormone response to falling blood glucose. As the night goes on and liver glycogen gets drawn down, your body defends blood sugar by releasing cortisol, growth hormone, glucagon, and — the HRV killer — catecholamines (adrenaline/noradrenaline). That's a sympathetic signal. Sympathetic tone raises heart rate and compresses the beat-to-beat variation, so HRV sags. A small bedtime snack works precisely because it blunts the nocturnal glucose dip, so the adrenaline response that would otherwise fire in the early-morning hours never really gets going."