Autophagy as cellular housekeeping
Autophagy is the regulated process by which cells break down and recycle damaged proteins and organelles. The term means self-eating, and the field was advanced enough for Yoshinori Ohsumi to receive the 2016 Nobel Prize in Physiology or Medicine for mapping its machinery. Autophagy runs at a baseline level constantly and is upregulated when nutrient sensing pathways signal scarcity.
Fasting lowers insulin and reduces signalling through mTOR while raising AMPK activity, conditions that tend to promote autophagy. The process ramps gradually rather than switching on at a single moment.
What the timing claims can and cannot say
Popular charts place sharp autophagy jumps at fixed fasting hours such as 16 or 24, but most precise measurements come from animal and cell studies, and human fasting timelines are inferred and individually variable. It is fair to say autophagy increases with prolonged fasting, but specific hour-by-hour percentages in humans should be treated as approximate and uncertain.
This is general educational information about a biological process, not a health protocol. Extended fasting carries real risks for some people and should be undertaken only with guidance from a qualified professional.