What happens after 16 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!

What's happening in your body

Insulin has been low for hours, which lets your fat cells release fatty acids that your muscles and heart burn directly ^1,2. Fat burning ramps up gradually as insulin falls; ketones are a later sign of it, not the starting point. Daily plans such as 16:8 repeat this switch every day without the strain of long fasts, which may be what drives many of fasting's effects ³.

Stage	Fat Burning (16 to 24 h)
Main fuel	Mostly stored fat
Insulin	Low

How you might feel

Many people feel lighter and more stable once hunger settles, and energy often comes back. If you don't, check your water first: even mild dehydration can bring headaches and low mood ⁴.

Does 16 hours make a difference?

For weight, a 16:8 window helps mainly by making it easier to eat less: on its own, it didn't beat three meals a day in a 12-week trial ⁵. Where it shines is as a simple, repeatable rhythm, especially with protein and strength training ⁶.

MYTH

You aren't burning fat until you're in ketosis.

FACT

Fat burning ramps up gradually as insulin falls; ketones are a later sign of it, not the starting point ¹.

Sources

1. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
2. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi.org/10.1056/NEJMra1905136
3. Mattson MP, Moehl K, Ghena N, et al. Intermittent metabolic switching, neuroplasticity and brain health. Nat Rev Neurosci. 2018;19(2):63-80. doi.org/10.1038/nrn.2017.156
4. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
5. Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Intern Med. 2020;180(11):1491-1499. doi.org/10.1001/jamainternmed.2020.4153
6. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. doi.org/10.1186/s12967-016-1044-0