Guide
16:8 intermittent fasting: when to start counting, and what it really does
16:8 is the most popular form of intermittent fasting: 16 hours without eating, then an 8-hour eating window. The idea is simple; the details are where people get stuck. When does the fast start? Does coffee break it? And what does it actually do? Here are clear answers, including the less exciting ones.
16:8 in one sentence
You go 16 hours without eating, then eat within an 8-hour window, every day or on the days you choose. In practice, the night does most of the work: if you finish dinner at 8 pm and eat again at noon, that's 16 hours.
When the 16 hours start
The fast starts with your last bite, not when you go to bed or when you remember to start a timer. Sleep counts. If you forgot to start your timer, simply set the start time to when you actually finished eating: what matters is the real time, not the button.
Nothing magic happens at exactly 16:00:00. If one day you stop at 15 hours, or go on to 17, the fast still counts.
What breaks a fast, and what doesn't
During the fasting hours, Johns Hopkins Medicine lists water and zero-calorie drinks such as black coffee and tea as permitted. Sparkling water and plain herbal tea fall in the same category.
Anything with calories ends the fast: milk in your coffee, sugar, juice, a snack, a smoothie. Drinks with sweeteners are debated; if you want to keep things simple, stick to water, black coffee and plain tea.
Choose a window that fits your life
The best window is the one you can keep. Common options:
- Noon to 8 pm: skip breakfast, keep family dinner. The most common choice.
- 10 am to 6 pm: a late breakfast and an early dinner.
- 8 am to 4 pm: for early risers who don't mind a light evening.
Social life matters: if Friday dinners are late, shift the window that day rather than giving up. Regularity helps more than perfection.
What research really shows
The honest answer is less spectacular than many apps suggest. In the TREAT trial, published in 2020 in JAMA Internal Medicine, 116 adults with overweight or obesity were randomised either to eat only between noon and 8 pm, or to eat three structured meals a day, for 12 weeks. The 16:8 group lost about 0.9 kg and the meals group about 0.7 kg: the difference between the two was not significant. The authors concluded that time-restricted eating alone was not more effective for weight loss than eating throughout the day.
A 2019 review in the New England Journal of Medicine by Rafael de Cabo and Mark Mattson describes promising effects of intermittent fasting on metabolism, many of them observed in animals, while human trials have mostly been short. Timelines you may see for “milestones” such as ketosis or autophagy come largely from that kind of research: treat them as rough orders of magnitude, not promises.
In other words, 16:8 is a framework. It helps some people eat more regularly and snack less in the evening; what you eat in the window still matters.
Who shouldn't fast
Johns Hopkins Medicine advises against intermittent fasting for children and teenagers under 18, and for people with a history of eating disorders, and warns that people with type 1 diabetes who take insulin risk dangerously low blood sugar while fasting. Pregnancy and breastfeeding, which increase energy needs, call for advice from your doctor or midwife. If you take regular medication or have a health condition, talk to your doctor before starting.
A timer that follows your real rhythm
FastClear is the fasting timer I make for exactly these details. You can correct the start time if you started late, go past your goal without the timer “ending”, and see your eating window counted down. A Live Activity keeps the fast on your lock screen, for free.
All protocols, the Live Activity, 30 days of history and the export are free; Premium adds your full history, statistics and widgets. No account, no ads, and FastClear gives no medical advice.
Questions people ask
When does a 16:8 fast start?
With your last bite of food, not when you go to bed. If you finish dinner at 8 pm, your fast ends at noon the next day. If you forgot to start your timer, set the start to when you actually stopped eating.
Does black coffee break a fast?
No. Johns Hopkins Medicine lists water and zero-calorie drinks such as black coffee and tea as permitted during fasting periods. Milk, sugar or anything with calories ends the fast.
Does 16:8 fasting help you lose weight?
Not necessarily more than other ways of eating. In the TREAT trial (JAMA Internal Medicine, 2020), 12 weeks of eating between noon and 8 pm did not lead to significantly more weight loss than three meals a day. It can help some people structure their eating; what you eat still matters.
Do I have to fast every day?
No. Many people fast on weekdays and relax at weekends, or shift their window on social evenings. A rhythm you can keep is better than a perfect one you give up.
Sources
- Lowe et al., « Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters (TREAT) », JAMA Internal Medicine 180 (11), 2020
- de Cabo, Mattson, « Effects of Intermittent Fasting on Health, Aging, and Disease », New England Journal of Medicine 381, 2019
- Johns Hopkins Medicine, « Intermittent Fasting: What is it, and how does it work? »