Fasting Timer

Chemically the body is still eating, even though the plate is empty.

• Insulin peaks after the meal; glucose from that meal is the main fuel.
• The surplus is stored as liver and muscle glycogen first, then as fat.
• Almost no fat is burned yet — high insulin directly suppresses lipolysis.

The liver takes over from food as the thing holding blood sugar steady.

• Insulin falls away and glucagon rises to take its place.
• The liver breaks down stored glycogen and releases it as blood glucose.
• Fat starts being used, but is not yet the main fuel.
• Anyone who sleeps eight hours wakes up here. This is simply what every morning is.

The liver starts turning fat into ketones — where fasting stops being just a skipped meal.

• Liver glycogen runs low, free fatty acids rise, and the liver begins making ketones.
• Reviews put this switch anywhere from 12 to 36 hours — not a fixed number. It depends on how much carbohydrate the last meal held and how much you moved during the fast.
• A 16:8 fast ends right at the start of this window.

The brain starts running on two fuels at once.

• Beta-hydroxybutyrate becomes a shared fuel for brain and muscle.
• Noradrenaline rises, which is why many people feel alert rather than drained.
• The "autophagy starts at 16 hours" figure everywhere online is not supported by human data. It is inferred from animal work; trials actually measuring autophagy in people are only now being run, at three-day fasts.

The liver stops dispensing reserves and starts manufacturing glucose instead.

• Blood glucose now comes almost entirely from gluconeogenesis — the liver building it from amino acids, glycerol and lactate.
• Growth hormone pulses get more frequent and larger, largely to protect muscle while no food is coming in.
• From here on, plain water is not enough — sodium, potassium and magnesium need attention.

The window where the research numbers move most — and they were measured in people.

• In healthy men fasting two days, 24-hour GH output rose about fivefold (78 → 371 μg/L·v) and the number of pulses went from 14 to 32 a day.
• Insulin sits at its lowest point of the whole cycle.
• Against the belief that fasting wrecks metabolism: over an 84-hour fast, resting energy expenditure rose about 14%, peaking on day 3, alongside a doubling of noradrenaline. The decline only comes with far longer fasts.

The body stops treating this as a missed meal and starts retuning whole systems.

• Reviews note that the hallmark endocrine adaptations only emerge after 48 to 72 hours; before that the changes are short-term ones.
• The thyroid down-shifts: over a 60-hour fast T3 fell from 1.73 to 1.36 nmol/L, reverse T3 rose from 0.30 to 0.44, TSH fell from 2.0 to 1.0 mU/L, while T4 was unchanged.
• Autophagy: this is the window human trials are actually measuring. There is no settled answer yet.

Past the point where doing this alone at home is a safe idea.

• IGF-1 can fall by up to 65% within 3-5 days, cortisol more than doubled over an 8-day fast, and testosterone dropped 17-36%.
• The concrete risks: electrolyte disturbance, orthostatic drops in blood pressure, loss of lean mass, and refeeding syndrome when you eat again.
• Breaking a long fast is riskier than the fast itself. Start small and light, not with a large meal.