What happens after 36 hours of fasting?

1. 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
2. 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
3. 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
4. 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!
5. 24 hKetosisFat is now your main fuel, and your liver turns more of it into ketones.
6. 36 hAutophagyCells may start clearing out old components, a natural recycling process.

What's happening in your body

Fat and ketones are your main fuel. As the fast goes on, ketones can become a major fuel for your brain ³, and growth hormone rises, which helps release fat and protect muscle ⁴.

Stage	Autophagy (36 to 48 h)
Main fuel	Fat and ketones
Insulin	Very low

What we know about autophagy

Autophagy is how your cells recycle worn-out parts, and it runs all the time at a low level ⁵. Fasting is one of its strongest natural triggers, clearly shown in animals ¹. In mice, starvation increased it in most tissues, but by different amounts in each organ ⁶. In people, the evidence is still early: eating within an early 6-hour window nudged up an autophagy gene in blood cells ².

MYTH

Autophagy switches on at exactly 16, 24 or 36 hours.

FACT

No study in people has found a switch-on hour. Autophagy rises gradually, differs between organs even in mice, and hasn't been timed in human tissues ^6,2. The Autophagy stage is an approximate guide, not a measurement.

How you might feel

Hunger is often lower than you'd expect, though energy can dip. Rest if you need to.

Sources

1. Bagherniya M, Butler AE, Barreto GE, et al. The effect of fasting or calorie restriction on autophagy induction: A review of the literature. Ageing Res Rev. 2018;47:183-197. doi.org/10.1016/j.arr.2018.08.004
2. Jamshed H, Beyl RA, Della Manna DL, et al. Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019;11(6):1234. doi.org/10.3390/nu11061234
3. Owen OE, Morgan AP, Kemp HG, et al. Brain metabolism during fasting. J Clin Invest. 1967;46(10):1589-95. doi.org/10.1172/JCI105650
4. Ho KY, Veldhuis JD, Johnson ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968-75. doi.org/10.1172/JCI113450
5. Levine B, Kroemer G. Biological Functions of Autophagy Genes: A Disease Perspective. Cell. 2019;176(1-2):11-42. doi.org/10.1016/j.cell.2018.09.048
6. Mizushima N, Yamamoto A, Matsui M, et al. In vivo analysis of autophagy in response to nutrient starvation using transgenic mice expressing a fluorescent autophagosome marker. Mol Biol Cell. 2004;15(3):1101-11. doi.org/10.1091/mbc.e03-09-0704