Live Fasting Timer
Track your fast in real-time and get notified when you hit key milestones
Fasting Milestones
Medical Disclaimer: This calculator provides estimates for educational purposes only and does not constitute medical advice. Intermittent fasting is not appropriate for everyone. Do not fast if you are pregnant or breastfeeding, under 18 years of age, have a history of eating disorders, or have been diagnosed with Type 1 diabetes. If you take medications (especially insulin or blood pressure medication), have any chronic health conditions, or are recovering from surgery, consult your physician before beginning any fasting protocol. If you experience chest pain, fainting, persistent dizziness, or heart palpitations during a fast, stop immediately and seek medical attention.
How to Use the Fasting Timer
- Set your fast start time by selecting when you had your last meal or clicking the start button.
- Choose your target fasting duration (16, 18, 20, 24 hours, or a custom length).
- Monitor your progress as the timer displays elapsed time and upcoming metabolic milestones.
- End your fast when you reach your target or when you decide to break it, and log the completed session.
Understanding Metabolic Phases During a Fast
A fast progresses through distinct metabolic phases. In the first 4 to 6 hours, your body digests and absorbs the last meal, using glucose as its primary fuel. Between 6 and 12 hours, glycogen stores begin to deplete and the liver increases gluconeogenesis. After 12 to 14 hours, fat oxidation becomes the dominant energy source and ketone production begins to rise.
Beyond 16 hours, autophagy activity increases as cells ramp up recycling of damaged proteins and organelles. At 18 to 24 hours, ketone levels climb further, providing an alternative fuel source for the brain and muscles. Growth hormone secretion also increases during this window, supporting tissue repair and fat metabolism.
Tracking these milestones in real time can provide motivation and context. Knowing that you have entered the fat-burning or autophagy zone helps you understand what is happening inside your body and make informed decisions about when to break your fast.
Key Takeaways
- The transition from glucose to fat burning typically occurs between 12 and 16 hours of fasting.
- Autophagy markers begin increasing around 14 to 16 hours and become more significant beyond 24 hours.
- Staying hydrated with water, electrolytes, and plain tea or coffee supports comfort during longer fasts.
- Real-time tracking helps build awareness of your fasting patterns and personal tolerance.
- Consistency across multiple fasts matters more than pushing for maximum duration in a single session.
Frequently Asked Questions
Does coffee or tea break a fast?
Plain black coffee and unsweetened tea contain negligible calories and do not significantly impact insulin levels. They are generally considered acceptable during a fast. Adding cream, sugar, or sweeteners may interrupt the fasted state depending on the amount.
What are the key metabolic milestones during a fast?
The major milestones are glycogen depletion at around 10 to 12 hours, the onset of significant fat oxidation at 12 to 14 hours, increasing autophagy at 16 to 18 hours, and elevated ketone production beyond 18 to 24 hours. Individual timing varies based on activity level, diet, and metabolic health.
How do I know if I am in ketosis during a fast?
Common signs include reduced appetite, a slight metallic taste, increased mental clarity, and a distinctive breath odor. For precise measurement, blood ketone meters provide the most accurate reading, with nutritional ketosis typically registering at 0.5 mmol/L or above.
Is it normal to feel hungry during a fast?
Yes. Hunger often comes in waves driven by ghrelin, which tends to spike at habitual meal times. These waves typically pass within 20 to 30 minutes. Staying busy, drinking water, and light activity can help manage hunger pangs.
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