So i was fasting in Fiji, and i decided to relook at a ton of old data i had on fasting and put some notes down.
The “Fasting Benefits Over Time” graphic shows that most of the marquee gains — stabilised glucose, surging ketones, autophagy and a burst of human-growth hormone — crescendo somewhere between 72 and 96 hours. Laboratory and clinical data agree: autophagy typically turns on after 24–48 h and appears to plateau by the three-day mark . From personal (and wearable-confirmed) experience, nights 1–2 bring a clear jump in sleep score and HRV; by night 3 the trend reverses, signalling that sympathetic stress is beginning to outweigh recovery.
3–4-day fasts
- Best for: autophagy, intestinal stem-cell activation, a clean ketone ramp, and modest weight loss.
- Physiology: insulin and IGF-1 fall, ketones rise steeply, and HRV often dips for the first time as cortisol rises .
- Practical take-away: stop here if your sleep score or overnight HRV plunges >10 % below baseline.
5-day fasts
- Added gain: deeper ketosis and further immune “re-boot”.
- Trade-off: lean-mass loss accelerates and metabolic-syndrome markers improve only transiently.
- Watch-out: devices often show a second sharp HRV fall and greater sleep fragmentation — a clue that the hormetic window is closing.
7-day fasts
- Research snapshot: a recent Nature Communications trial found seven-day water fasts halved muscle glycogen and cut VO₂ max by 13 % while preserving strength . Resting metabolic rate and thyroid output begin to throttle down, foreshadowing the “lock-up” sensation many fasters report .
- Recommendation: reserve for therapeutic contexts and cycle sparingly; monitor HRV daily and be ready to re-feed once values trend below baseline for two consecutive nights.
10 days and beyond
Although extended fasts can drive ketones higher and strip additional weight, studies show disproportionate lean-mass loss, rising stress hormones and progressively lower HRV. Subjectively I’ve found cognitive sharpness and sleep quality suffer after day 8, confirming the chart’s downward slope for key benefits while stress biomarkers mount.
Bottom line: For most healthy adults, three to four days strike the best balance of cellular renewal and manageable stress. Use objective metrics — sleep score, overnight HRV and resting heart rate — to personalise the finish line; when recovery markers tumble, you’ve likely moved past benefit into biological debt.