Fasting Timer & Tracker

Set your fasting window (16:8, 18:6, OMAD, custom) and start the timer. The display shows hours elapsed, hours remaining, and which metabolic phase your body is likely in: glycogen depletion in the first 12 hours, ketosis onset around hour 16, autophagy ramping past 24, growth hormone elevation through 36-48 for extended fasts.

The phase markers are based on average research findings, not a personal physiology model. Individual variation is huge — someone metabolically flexible enters ketosis hours faster than someone newly fasting. Treat the markers as rough orientation, not medical guidance.

For multi-day fasts, talk to a doctor first if you take medications, manage diabetes, or have any condition affected by long periods without food. The timer just tracks time; it doesn't know your medical context.

The biology behind the phase markers: hours 0-4 your body uses just-eaten glucose. Hours 4-12 depletes liver glycogen — the postabsorptive state. Past 12 hours, glycogen is gone and the liver shifts to producing ketones from fat (gluconeogenesis and ketogenesis ramp up). At ~16 hours blood ketones typically rise above 0.5 mmol/L (the nutritional ketosis threshold). Autophagy — cells recycling damaged components — increases gradually starting around 16-24 hours, with most research suggesting meaningful elevation past 24 hours and progressive increases through 48-72 hours. Growth hormone secretion increases substantially past 24 hours and stays elevated through extended fasts. Insulin drops dramatically by 24 hours, which is part of why fasting affects insulin sensitivity.

Worked example: you start a 16:8 fast at 8pm after dinner. The timer counts forward. By 8am the next morning (12 hours), liver glycogen is mostly depleted but you might not feel ketosis yet. By 12pm (16 hours), if you're metabolically flexible, ketones are rising and hunger plateaus. Eating window opens noon to 8pm. Repeat daily. For a 24-hour fast (eat at 8pm, next meal 8pm next day), you cross the autophagy threshold; for 36-48 hour fasts (less frequent, more taxing), you hit the deeper phases.

What the timer can't tell you: whether you should be doing this. Time-restricted eating is broadly safe for healthy adults; extended fasts are riskier for people with diabetes (especially Type 1), eating disorder history, pregnancy, or anyone on medications that require food. Refeeding after long fasts can cause refeeding syndrome — electrolyte disturbances that are dangerous if not managed. Black coffee, plain tea, and water are widely considered to not break a fast metabolically. Anything caloric does. Sweeteners are debated; the research is mixed on whether non-nutritive sweeteners affect insulin response.