To trigger autophagy, fasting typically needs to last at least 16 to 18 hours. Autophagy begins as the body depletes glucose and insulin levels drop, entering a metabolic state where cells start recycling damaged components. While initial autophagy can begin within this 16-18 hour window, research shows it reaches higher activity and peaks between 36 to 72 hours of fasting. Extended fasts beyond 48 hours can promote a more sustained autophagic response, enhancing cellular repair and health. However, prolonged fasting should be approached cautiously, ideally under medical supervision, especially for those with medical conditions or on medications. Intermittent fasting (such as 16/8 or 17-19 hour fasts) can initiate autophagy regularly during fasting windows, while longer fasts (2-4 days) maximize autophagy benefits but require careful planning. In summary:

• Autophagy begins approximately after 16-18 hours of fasting.
• Autophagy activity increases and peaks typically between 36-72 hours.
• Extended fasting over 48 hours sustains autophagy for deeper cellular cleanup.
• Shorter intermittent fasts can trigger autophagy repeatedly but at moderate levels.
• Safety and individual health factors are important to consider before extended fasting.

Practical advice is to start with overnight fasts around 16-18 hours and extend fasting durations gradually if seeking greater autophagy benefits, with professional guidance recommended for fasts beyond 24 hours.