Hour-by-hour timeline of impact 36-hour fast has on body as jaw dropping simulation shows major effects
ArchiveFirst published May 20, 2025. It may not reflect later developments.
Fasting for 36 hours may sound extreme, but it’s been linked to a range of health benefits—if done safely and with hydration. Experts stress that it’s not for everyone and medical advice is a must before trying it.
After 4 hours, insulin drops and your body starts burning stored sugar (the “anabolic phase”). By hour 12, fat begins to burn through a process called ketosis. At 16 hours, autophagy kicks in—your cells begin cleaning out damaged parts. This boosts cell health and energy.
By 24 hours, inflammation decreases and insulin sensitivity improves. At 30 hours, growth hormone spikes, preserving muscle and accelerating fat loss. At 36 hours, you reach peak autophagy—clearing dead cells and regenerating tissue.
“There [are] a lot of proposed benefits to [running on fats]. But… we don’t see dramatic health benefits, certainly in the short term,” says Professor James Betts of the University of Bath.
While fasting may offer a “reset,” science is still catching up. Always consult a doctor before starting.
