Short answer. Start with a 12 to 14 hour overnight fast, keep it until it feels easy, often a week or two, then move to 16:8 if you want it. On its own, the eating window does little for weight: in trials it added between nothing and about 2 kg of weight loss over three to twelve months, and what people ate mattered more. If you're pregnant or breastfeeding, a child or teenager, underweight, have had an eating disorder or take diabetes medicines that lower blood sugar, talk to a doctor first.
What intermittent fasting is
Intermittent fasting is a pattern of eating, not a diet. You eat within a set window each day and take nothing with calories outside it. Plans are named by their hours: 14:10 means 14 hours without food and a 10-hour eating window; 16:8 means 16 and 8.
The pattern says nothing about what you eat inside the window, which is why it fits most ways of eating and fixes none of them. Your body changes over the hours of a fast; what happens when you fast, hour by hour lists the stages with the study behind each one.
How to start intermittent fasting: your first plan
Start with a fast you will barely notice, then shorten the eating window in steps: a 12 to 14 hour overnight fast until it feels easy, often a week or two, then 16:8 if you want it. That sequence is common practice and expert advice, not a trial result. No trial we found sets the timescale, and you can stay at any step for as long as you like.
The experts say the same in their own words. Courtney Peterson, Associate Professor of Nutrition at Harvard T.H. Chan School of Public Health, recommends an eight-hour or shorter window as the goal, but says you may need to start with ten hours and work down (Harvard T.H. Chan School of Public Health, 24 September 2025). Harvard Health Publishing advises reducing the window slowly, «over a period of several months» (Harvard Health Publishing, undated, read 2026-11-07). Cleveland Clinic calls the «16/8 or 14/10 method» «a safer bet for many people who are interested in trying intermittent fasting for the first time» (Cleveland Clinic, 26 January 2026).
| Step | Plan | Example day | Move on when |
|---|---|---|---|
| 1 | 12:12 or 13:11, an overnight fast | Last bite 8 pm, breakfast 8 or 9 am | After a few days, if you barely noticed it |
| 2 | 14:10 | Last bite 8 pm, first meal 10 am | After a week or two, once hunger is a passing thing |
| 3 | 16:8 | Last bite 8 pm, first meal noon | Stay here if it suits you; longer fasts are a separate decision |
Skip step 1 if you already go 12 hours between dinner and breakfast most nights. Many people do, without calling it anything.
If you track it, every step is a free daily plan in Bare Fast, the app we make: 13:11 for step 1, 14:10 for step 2, then 16:8. Its shortest plan is 13:11, so a 12-hour fast needs only a clock.
Which hours to pick
Anchor the fast to sleep. Most of a 12-hour fast passes while you’re asleep, so a fast from after dinner to breakfast costs you one evening snack. Then set the end of the window around the evening you actually have: dinner with your family at 7 pm means the fast starts at 8.
Keep the same window every day. Cleveland Clinic’s advice is to keep a similar eating and fasting window on the days you fast (Cleveland Clinic). A reminder at the same time each day helps: in Bare Fast, make a plan your usual one and set a start time and a daily reminder.
What the first two weeks feel like
Hungry, often in waves. Australia’s healthdirect, a government-funded health advice service, says it plainly: «When you first start intermittent fasting, you will feel hungry», and you may also get headaches (healthdirect, reviewed December 2025). Harvard Health Publishing adds lethargy, crankiness and constipation, and says easing into the plan helps your body adjust (Harvard Health Publishing).
A wave of hunger usually passes whether or not you eat; a glass of water, a walk or a task is often enough to outlast it. It tends to get easier, though the sources above put no number of days on that. If the hunger comes with weakness, or you can’t concentrate or get through your day, that is not a wave: see the signs to stop below.
What you can have during the fast
Water, black coffee and plain tea. Cleveland Clinic: «You can drink water and other beverages, like black coffee and tea, during fasting periods», and «to maintain a fasting state, you must not consume any foods or drinks with calories» (Cleveland Clinic). healthdirect notes that water is fine at any time (healthdirect).
That rule settles most «does X break a fast» questions: milk, sugar, juice or a biscuit ends the fast. The grey areas, such as a splash of milk in your coffee, are in the questions at the end of the 16:8 fasting schedule guide.
The eating window is not a licence
An eating window on its own does little for weight; what you eat inside it does most of the work. Trial reports call a daily eating window «time-restricted eating».
| Trial or review | Who, how long | What was compared | Result |
|---|---|---|---|
| Lowe et al., JAMA Internal Medicine, 2020 (TREAT) | 116 adults with overweight or obesity, 12 weeks | Eat freely from noon to 8 pm, no calorie target, versus three meals a day | Difference between groups 0.26 kg, not statistically significant (small enough to be chance) |
| Liu et al., New England Journal of Medicine, 2022 | 139 adults with obesity, 12 months | Calorie restriction with an eating window versus calorie restriction alone | 8.0 kg versus 6.3 kg lost; the 1.8 kg gap was not statistically significant |
| Semnani-Azad et al., BMJ, 2025 | 99 randomised trials, 6,582 adults, healthy and with health conditions | Every intermittent fasting type versus continuous calorie restriction | Similar benefits; time-restricted eating did not beat calorie restriction |
| Jamshed et al., JAMA Internal Medicine, 2022 | 90 adults with obesity, 14 weeks | Calorie restriction with an early 8-hour window (7 am to 3 pm) versus calorie restriction with a window of 12 hours or more | 2.3 kg more lost with the early window |
| Fernandes-Alves et al., Nutrition Reviews, 2026 | 30 randomised trials, 1,341 adults with overweight or obesity | Time-restricted eating versus controls; the 1.46 kg figure is from the subset of trials whose controls ate the same calories | 1.46 kg more lost on average with the window |
The first three found that the window adds little on its own. The TREAT authors concluded that time-restricted eating «is not more effective in weight loss than eating throughout the day». The last two found it may add a little: 2.3 kg with an early window over 14 weeks, and about 1.5 kg in the 2026 meta-analysis, with a wide margin of uncertainty (95% CI 0.26 to 2.65 kg).
Put together: in trials mostly of adults with overweight or obesity lasting three to twelve months, the window itself added somewhere between nothing and about 2 kg, and what was eaten inside it mattered more. None of this is a promise about you. Harvard Health Publishing also names a practical catch: «a strong biological push to overeat following fasting periods» (Harvard Health Publishing). If the window turns into a feast, the fast has changed nothing.
Sleep, exercise and social life
Fit the window to your life, not your life to the window.
Sleep. Finish eating a while before bed rather than right at it, and if you sleep badly after starting, move the whole window earlier rather than eat less. The one trial above that reported sleep (Jamshed and colleagues, 14 weeks) found similar sleep outcomes in both groups; the rest is practical advice, not a finding.
Exercise. Keep light exercise where it is, and place hard sessions near the window so you can eat afterwards; the 16:8 guide covers training in the fasting hours. One trial gives a reason to look after muscle. TREAT (Lowe et al., JAMA Internal Medicine, 2020) scanned 50 of its participants with DXA, an X-ray scan that splits body weight into fat, lean tissue and bone. The 25 on the noon-to-8-pm window lost 1.70 kg in 12 weeks, and about 65% of it, 1.10 kg, was lean mass; only 0.51 kg was fat. The authors note that lean tissue is usually 20 to 30% of the weight people lose. Their appendicular lean mass index, the lean mass of the arms and legs (mostly muscle) divided by height squared, fell compared with the group eating three meals a day.
The trial did not measure protein, and the scan did not account for how much water the muscle held, so read this as a caution from one small group, not a verdict. The authors’ advice is to eat enough protein inside the window, and keeping whatever strength work you already do makes sense too.
Social life. A dinner at 9 pm does not break a habit. Shift the window that day, start the next day’s first meal later, and carry on. If the late evening falls on the same day each week, plan for it: Bare Fast Pro’s weekly Schedule can put a shorter plan on that day.
Who should not start, or should talk to a doctor first
Australia’s healthdirect says you should not do intermittent fasting if you are underweight, have or have previously had an eating disorder, are pregnant or breastfeeding, are a child or teenager, are an older adult, or take daily medicine that must be taken with food (healthdirect). Harvard and the Cleveland Clinic name much the same groups.
If you take diabetes medicines that lower blood sugar, talk to whoever prescribes them first. In a 12-week trial of 41 adults with type 2 diabetes, two low-calorie fasting days a week roughly doubled the rate of hypoglycaemia, even though doses were reduced (Corley et al., Diabetic Medicine, 2018). Who should not do intermittent fasting has all four medical lists, the diabetes detail and what to ask your doctor.
Signs to stop
Stop and speak to your doctor if you feel weak, anxious or irritable, can’t concentrate, or can’t do your usual activities (healthdirect). A wave of hunger usually passes; feeling unable to get through your day does not. If you take diabetes medicines that lower blood sugar and notice any sign of a low, treat it the way you have been told to, and tell your doctor before you fast again. If a single fast is going badly, end it: nothing is lost but a few hours.
Tracking without pressure
A 12-hour overnight fast needs no app: last bite at 8 pm, breakfast at 8 am, and a clock does the job. A timer earns its place when the window shortens. At 14:10 or 16:8 it answers «when can I eat» without arithmetic, and it records the days, so after a week or two you can see whether the plan fits.
What a tracker should not do is nag. A streak that breaks on a rest day, or an app that calls an early end a failure, turns a habit into a debt; what barehabits is about explains why our apps work the other way. In Bare Fast, a rest day or a pause does not break the streak, and an early end reads «of 16 h», never «failed».
Common mistakes
- Starting at 16:8 on day one. A week or two at 12 to 14 hours first costs nothing.
- Treating the window as a licence. The trials above say the window adds little on its own.
- Eating too little inside the window. Harvard Health Publishing warns of a strong push to overeat after a fast; a half-empty window gives that push more room.
- Milk or sugar in the «fasting» coffee. Anything with calories ends the fast.
- Moving medication times without asking. Talk to whoever prescribes them first; Cleveland Clinic says the same.
- Chasing the streak or a longer fast. A fast ended early is a fast, and a 24-hour fast is a different decision with its own risks: how long a fast should be covers the ladder from 12 to 72 hours.
Common questions
Which intermittent fasting schedule should a beginner start with?
A 12 to 14 hour overnight fast, kept until it feels easy, then 16:8 if you want it. Cleveland Clinic calls 16:8 or 14:10 a safer bet for a first attempt, and the 16:8 fasting schedule guide compares 14:10, 16:8 and 18:6 when you are ready.
Can I drink coffee or tea while fasting?
Yes. Black coffee and plain tea are fine, and water at any time (Cleveland Clinic). Anything with calories, including milk and sugar, ends the fast.
Will I lose weight if I only change when I eat?
In trials, somewhere between nothing and about 2 kg over three to twelve months. In the 12-week TREAT trial of 116 adults (Lowe et al., 2020), people eating freely in a noon-to-8-pm window lost no more than people eating three meals a day. What you eat inside the window does most of the work.
Do I need an app to start intermittent fasting?
No. A 12-hour overnight fast is a bedtime and a breakfast. An app helps once the window shortens and you want the end time and the days recorded without thinking about them. Before you pay for one, check what the free version covers and how the trial ends.
What if I miss a day of intermittent fasting?
Nothing happens. The next fast starts at your usual last bite, and the pattern carries on. The same window on most days matters more than any single day.
This post is not medical advice. Talk to your doctor before you change how you eat, especially if you take medication or have a medical condition.