Most intermittent fasting plans involve fasting for 12 to 20 hours per day, with 16 hours being the most popular starting point. The number you choose depends on your experience level, your schedule, and what you’re trying to get out of it. Here’s how the different fasting windows break down and what actually happens in your body at each stage.
The Most Common Fasting Schedules
Intermittent fasting is defined by two numbers: hours fasting and hours eating. The first number is always the fasting window.
- 12:12 — 12 hours fasting, 12 hours eating. This is essentially just not snacking after dinner and before breakfast. If you finish eating at 8 p.m. and have breakfast at 8 a.m., you’re already doing it.
- 14:10 — 14 hours fasting, 10 hours eating. A common version is eating between 9 a.m. and 7 p.m. This is often recommended for beginners or for people who find 16 hours too restrictive.
- 16:8 — 16 hours fasting, 8 hours eating. The most widely followed protocol. A typical schedule is eating between 11 a.m. and 7 p.m., which means skipping breakfast or shifting it later.
- 20:4 — 20 hours fasting, 4 hours eating. A more aggressive approach that compresses all food intake into a late afternoon or evening window, usually fitting in one large meal and a snack.
- OMAD (23:1) — 23 hours fasting, roughly 1 hour eating. All daily calories come from a single meal. This is the most extreme daily fasting protocol.
There are also non-daily approaches. Alternate-day fasting cycles between regular eating days and fasting days, where you either eat nothing or limit intake to about 25% of your normal calories. The 5:2 method applies a similar idea but limits fasting to just two days per week.
What Happens in Your Body Hour by Hour
The reason fasting hours matter is that different metabolic processes kick in at different points during a fast. Your body doesn’t flip a single switch the moment you stop eating. Instead, it moves through a gradual sequence.
For the first several hours after your last meal, your body runs on glucose from the food you just ate. Once that’s used up, it taps into glycogen, the stored form of glucose kept in your liver and muscles. This supply typically lasts through the first 12 to 18 hours of a fast, which is why even a basic 12-hour overnight fast gives your metabolism a chance to fully process the day’s food before you eat again.
Somewhere around 18 to 24 hours, glycogen stores in the liver become depleted and your body starts breaking down fat for energy, producing compounds called ketone bodies. This metabolic shift is called ketosis. However, fasts shorter than 24 hours generally won’t reach full ketosis unless you’re also eating very few carbohydrates during your eating window. So if you’re doing a standard 16:8 fast, you’re unlikely to enter deep ketosis, though you are burning more fat than you would if you were eating around the clock.
Growth hormone output rises meaningfully during fasting. One study found that a 24-hour water-only fast increased growth hormone levels by 5-fold in men and 14-fold in women. This spike was independent of weight loss, meaning it’s a direct response to the fasting state itself, not just a side effect of eating less.
Autophagy, the process where your cells break down and recycle damaged components, appears to ramp up between 24 and 48 hours of fasting based on animal studies. The exact timing in humans isn’t well established, so claims that a 16-hour fast triggers significant autophagy are likely overstated.
Why Overnight Fasting Hours Count
One of the most consistent findings in fasting research is that aligning your fast with sleep amplifies the benefits. A randomized controlled trial published in the journal Arteriosclerosis, Thrombosis, and Vascular Biology tested what happened when overweight adults extended their overnight fast by just 3 hours (finishing their last meal at least 3 hours before bed, reaching a total fast of 13 to 16 hours). Over 7.5 weeks, the extended fasting group showed lower nighttime heart rate, lower nighttime cortisol, better blood sugar control the following morning, and improved blood pressure dipping during sleep compared to the control group eating on their normal schedule.
Notably, this didn’t disrupt sleep. Total sleep time, sleep efficiency, and time spent in each sleep stage were unchanged. The gains came purely from giving the body more hours without food while it was already at rest. This suggests that even if you’re not ready for a 16-hour fast, simply stopping food intake a few hours earlier in the evening and reaching 13 or 14 hours of fasting overnight can produce measurable cardiovascular and metabolic improvements.
Choosing the Right Number of Hours
If you’ve never fasted intentionally, starting at 12 hours is a reasonable entry point. Most people can do this by simply setting a cutoff time for evening eating and not snacking before bed. From there, pushing to 14 hours is a small adjustment that still fits a conventional meal schedule.
The 16:8 protocol is where most people settle long-term. It’s restrictive enough to create a meaningful fasting window but flexible enough to maintain socially. Skipping breakfast or pushing it to late morning is the easiest way to hit 16 hours for most schedules. If you eat dinner at 7 p.m., your first meal the next day would be at 11 a.m.
Protocols of 20 hours or more are significantly harder to sustain. OMAD requires eating your entire day’s nutrition in a single sitting, which can make it difficult to consume enough protein, fiber, and micronutrients. These longer fasts can also cause lightheadedness, irritability, and difficulty concentrating, especially in the first few weeks.
Longer Fasts: 24 to 48 Hours
Some people practice periodic 24-hour or 48-hour fasts once or twice a month rather than fasting daily. These longer fasts push the body further into fat-burning and may trigger more autophagy, but they carry more risk. People with type 1 diabetes, low blood pressure, a history of eating disorders, or those who are pregnant or nursing should not attempt extended fasts. Anyone taking medications for blood pressure, blood sugar, or blood clotting should check with their prescriber first, since fasting can change how these drugs behave in the body.
For most healthy adults exploring intermittent fasting, the 14 to 18 hour range offers the best balance of metabolic benefit, sustainability, and safety. The “right” number of hours is ultimately the one you can maintain consistently without it taking over your relationship with food.