Steady
The truth about intermittent fasting
What intermittent fasting actually does. And what it doesn't.
The mechanism is simple and slightly boring. The dangers are real but specific. And most of what fasting apps promise about a 16 hour window is happening somewhere past hour 24.
The whole mechanism in one paragraph
After a meal, your body runs on the sugar from that meal and stores the surplus, some as glycogen in the liver and muscles, some as fat. As the hours without food pass, it uses the meal up, then draws down glycogen, then leans increasingly on stored fat for fuel. That handover from sugar to fat is what intermittent fasting is. There is no mystery in it and no switch that fasting alone flips. Everyone does this every night. A fasting window just makes the night longer.
A plain timeline
These are general descriptions of a typical window. They are not readings from you, and no app can take them. Steady uses the same labels on its timer and says the same thing there.
- 0hJust ateStill working through the last meal.
- 3hWinding downDigestion is wrapping up and things are settling.
- 8hOn reservesAround now the body usually turns to its glycogen stores.
- 12hThe switchReserves run low around here, and fat starts doing more of the work. This is also where the hard hours tend to begin.
- 14hRunning on fatHunger tends to come in waves through this stretch.
- 16hThe good partThe stretch most people come to fasting for. For many, the second half feels easier than the first.
- 18hCruisingPast the hard part. For many people hunger settles from here.
- 20hFull burnThe far end of what Steady asks of anyone.
What does not happen at 16 hours
Autophagy and true nutritional ketosis need roughly 24 hours or more without food. A 16 hour window gets you into the fat-burning stretch, which is real and worth having, but it does not get you to the cellular clean-up and deep ketone states that fasting marketing likes to attach to every window. Steady tops out at 20 hours, so it does not reach them either, and we would rather say so than inflate the numbers.
The practical consequence: if an app tells you autophagy has begun at hour 14, it is reading from a script, not from you. Nothing on a phone can measure it.
Why the weight loss is mostly about eating less
Intermittent fasting helps people lose weight mainly by giving them a structure that leads to eating less across the week. Fewer hours to eat in, fewer late snacks, one meal quietly dropped. Trials that compare time-restricted eating with ordinary calorie reduction mostly find similar results for similar intake. The window is a tool for eating less that some people find easier to stick to than counting. It is not a mechanism that burns extra on its own.
This is good news and bad news. The good news is that it works for a lot of people precisely because it is simple. The bad news is that if you eat the same amount inside a shorter window, not much changes, and the hunger you put up with bought you nothing.
What it does to mood and energy
This is the part almost nobody warns you about, and it is the part that ends most fasts. Somewhere in the second half of a window, tiredness, hunger and low mood tend to arrive together. Each makes the others worse. The stack lands at roughly the same point for a given person and, for many, lifts again not long after. The lift is the thing to know about, because from inside the hard hour it is invisible. We wrote a separate piece on why people stop, because that hour deserves its own page.
When intermittent fasting is dangerous
For most healthy adults, a 12 to 16 hour overnight window is not dangerous. The risk concentrates in specific situations, and they are specific enough to list.
- You take medication for diabetes or blood sugar, or any medication whose timing depends on meals. Insulin and sulfonylureas can cause dangerous low blood sugar during a fast; SGLT2 inhibitors carry a risk of ketoacidosis even with normal glucose.
- You are pregnant, breastfeeding or trying to conceive.
- You have, or have had, an eating disorder. A fasting window can become a way of hiding restriction, and no app can tell the difference.
- You have a heart, kidney or liver condition, or low blood pressure.
- You are under 18 or over 70.
- You are recovering from illness or surgery.
Stop and eat immediately, whatever the hour, if you feel shaky, dizzy, confused, faint, sweaty for no reason, or notice a racing heart or blurred vision. These are not "push through" symptoms. Steady's check-in has a row for exactly these, and tapping any of them ends the coaching and tells you to eat.
The ordinary side effects
- Hunger in waves. Normal. A wave usually passes; water and a change of scene help.
- Headaches in the first week. Common, often from less caffeine or less salt than usual. They tend to fade.
- Irritability and low mood. Common, and the main reason people quit. See above.
- Poor sleep. Usually when the eating window ends late, or when coffee creeps into the fast in the evening.
- Feeling cold. Some people notice it in longer windows. Not dangerous on its own, but worth noting.
What a sensible window looks like
Around hour 12 the body typically turns from the last meal to its reserves. A window that ends before that is skipping breakfast. One that reaches it is a fast. Most people already go nine or ten hours overnight without thinking about it, so 12 is the floor and 14 is a real first step. Pushing straight to 16 or 18 on day one, failing once and deciding fasting is not for you is the most common way to waste the attempt. Start one step above where you already are, and raise it when that feels routine.
Questions people ask
Is intermittent fasting dangerous?
For most healthy adults a 12 to 16 hour overnight window is not dangerous. It becomes a medical question if you take medication whose timing depends on meals, especially insulin, sulfonylureas or SGLT2 inhibitors, if you are pregnant or breastfeeding, under 18 or over 70, have a heart, kidney or liver condition or low blood pressure, or have a history of disordered eating. In those cases talk to a doctor before starting.
What are the side effects of intermittent fasting?
The common ones are hunger in waves, tiredness, irritability or low mood, headaches in the first week, and poor sleep if the eating window ends late. Warning signs that mean stop and eat now are shakiness, dizziness, confusion, a racing heart, blurred vision or feeling faint.
Does 16:8 intermittent fasting work for weight loss?
It can, but mostly because it helps some people eat less across the week. Trials comparing time-restricted eating with ordinary calorie reduction generally find similar weight loss. If the window does not change how much you eat overall, it will not do much on its own.
Does a 16 hour fast trigger autophagy or ketosis?
Not meaningfully. Autophagy and true nutritional ketosis need roughly 24 hours or more without food. A 16 hour window shifts the body toward using stored fat for fuel, which is real and useful, but the deeper processes that fasting apps like to advertise are further out than most people ever fast.
What actually happens hour by hour during a fast?
Roughly: the first few hours digest the last meal, by around hour 8 the body is drawing on stored glycogen, around hour 12 those reserves run low and fat starts doing more of the work, and from 14 to 20 hours the body is running mainly on fat. These are general descriptions of a typical window, not measurements of you.
Get through the hard hours.
Steady is an intermittent fasting coach app for iPhone. It works out which hour tends to beat you, checks in just before it, and tells you to eat when you should. Your data never leaves your phone.