Short answer. 16:8 fasting means eating inside the same 8 hours each day and taking only water, black coffee or unsweetened tea in the other 16. In trials of 8 weeks to 12 months it changed weight a little when people ate freely and, pooled across reviews, added little or nothing to ordinary calorie cutting; it helps when the shorter window makes you eat less. Pick the 8 hours that fit your day and keep them steady.
What 16:8 fasting is
16:8 fasting means you eat inside the same 8 hours each day and take nothing with calories in the other 16. The British Heart Foundation describes it as the pattern «where you only eat during an eight-hour window and fast for the other 16 hours» (British Heart Foundation, Behind the headlines: intermittent fasting, 2024). Researchers call it time-restricted eating, and the trials below use that term.
In the 16 hours you can have water, carbonated water, black coffee and unsweetened tea, and nothing with calories. That is the Cleveland Clinic’s rule (Cleveland Clinic, What Is Intermittent Fasting?, 2026).
You don’t need an app for 16:8, but a timer keeps the window honest. 16:8 is one of the six free daily plans in Bare Fast, the app we make.
If you have never fasted at all, start with how to start intermittent fasting, which builds up from 12 hours over the first two weeks.
Your 16:8 schedule: choosing the 8 hours
Pick the 8 hours that hold the meals you would not want to give up, then keep them steady. The clock time matters less than the sameness: every daily trial below set one window and held it for the whole study.
| Window | You fast from | Fits people who | A trial that used it |
|---|---|---|---|
| Early, 8 am to 4 pm | 4 pm to 8 am | eat breakfast, finish work early, go to bed early | Liu 2022 (with a calorie limit) |
| Standard, 12 pm to 8 pm | 8 pm to 12 pm | skip breakfast anyway, eat dinner with family or friends | Lowe 2020 |
| Late, 1 pm to 9 pm | 9 pm to 1 pm | sleep late, eat late, train in the afternoon | Close to Moro 2016 (meals at 1 pm, 4 pm and 8 pm) |
Harvard Health’s example is 9 am to 5 pm (Harvard Health, Time to try intermittent fasting?, 2026) and the Cleveland Clinic’s 11 am to 7 pm; any 8 hours count as 16:8 if they are the same 8 hours most days.
If you work shifts
Anchor the window to your sleep, not to the clock, and consider 14:10 before 16:8. The one shift-work trial we found used a 10-hour window: 137 firefighters on 24-hour shifts followed it for 12 weeks, and among those who started with raised risk markers, HbA1c (long-term blood sugar) and diastolic blood pressure fell compared with standard care (Manoogian et al., Cell Metabolism, 2022). We found no trial of an 8-hour window in shift workers.
What happens during the 16 hours
For the first few hours after your last meal, your body is still digesting it and storing what it does not need right away. After that it runs on what it stored earlier, first from the liver and then increasingly from fat. The hour-by-hour account, with the study behind each phase, is on what happens when you fast, hour by hour. In Bare Fast, the ring shows the phase you are in and the time left.
16:8 fasting results: what the trials found
On its own, with nobody counting calories, 16:8 led to small weight changes, and in the largest trial it did no better than three meals a day. Added to calorie counting, the window made no significant difference over 12 months in one trial and added 2.3 kg over 14 weeks in another. That is the trial evidence for 16:8 and the windows next to it: four randomised trials and one pilot, 23 to 139 people, none longer than a year.
| Trial | People, length | Compared | Result |
|---|---|---|---|
| Lowe 2020, JAMA Internal Medicine | 116 adults with overweight or obesity, US, 12 weeks | 12 pm to 8 pm vs three structured meals a day, no calorie target | 0.94 vs 0.68 kg lost; net 0.26 kg, not significant |
| Gabel 2018, Nutrition and Healthy Aging (pilot) | 23 adults with obesity, US, 12 weeks | 10 am to 6 pm vs a historical control (not randomised) | 2.6% of body weight lost; about 341 kcal a day less |
| Cienfuegos 2020, Cell Metabolism | 58 adults with obesity, US, 8 weeks | 18:6 (1 pm to 7 pm) or 20:4 (3 pm to 7 pm) eating freely vs control | About 3% lost on both; about 550 kcal a day less |
| Jamshed 2022, JAMA Internal Medicine | 90 adults with obesity, 80% women, US, 14 weeks | 7 am to 3 pm vs 12 hours or more, both on a calorie limit | 2.3 kg more for the early window (P = 0.002) |
| Liu 2022, New England Journal of Medicine | 139 adults with obesity, China, 12 months | 8 am to 4 pm plus a calorie limit vs the limit alone | 8.0 vs 6.3 kg lost; net 1.8 kg, not significant |
Without a calorie target
In the largest trial of 16:8 on its own, the 0.26 kg difference over 12 weeks was not significant, and the 16:8 group lost a little more lean mass in the arms and legs (Lowe et al., JAMA Internal Medicine, 2020). The authors’ conclusion: «Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day.»
Two smaller studies from one research group found more, and in both, people ate less without being asked to: about 341 kcal a day less in a 12-week pilot (Gabel et al., Nutrition and Healthy Aging, 2018) and about 550 kcal in an 8-week trial of 18:6 and 20:4, which had no 16:8 arm (Cienfuegos et al., Cell Metabolism, 2020).
With a calorie target
When both groups count calories, the two trials disagree. Over 12 months, adding an 8 am to 4 pm window to a calorie limit gave 1.8 kg more weight loss, a gap that was not significant (Liu et al., New England Journal of Medicine, 2022). Over 14 weeks, adding a 7 am to 3 pm window gave 2.3 kg more (Jamshed et al., JAMA Internal Medicine, 2022). Whether the early timing or something else explains the gap is not settled.
What the reviews say
Pooled across every fasting pattern, intermittent fasting matches ordinary calorie cutting. A 2025 network meta-analysis of 99 randomised trials with 6,582 adults found that every fasting strategy reduced weight compared with eating freely, and that against calorie cutting only alternate-day fasting showed a benefit, of 1.29 kg (Semnani-Azad et al., BMJ, 2025). One 2026 review leans the other way: across 30 studies with 1,341 participants, time-restricted eating edged people eating the same calories without a window by 1.46 kg (Fernandes-Alves et al., Nutrition Reviews, 2026). Read together, the window’s effect beyond eating less is somewhere between nothing and about one and a half kilograms over the months a trial runs.
None of this reaches beyond a year: Harvard Health notes that «most trials last for 12 months or less» (Harvard Health, 2026). The 2024 headlines that linked an 8-hour window to cardiovascular death came from a study that, the British Heart Foundation pointed out, «is also observational, meaning it cannot show cause and effect, only a link between two factors» (British Heart Foundation, 2024).
Why 16:8 works when it works
16:8 is a structure that helps some people eat less, not a metabolic trick. In the two studies above where the window worked, people ate 341 to 550 kcal a day less without being asked to. Where nobody had a calorie target and the window did not beat three meals a day (Lowe 2020), there was no extra loss either.
The window is harder to keep than it sounds. Harvard Health warns that «there’s a strong biological push to overeat following fasting periods» (Harvard Health, Not so fast: Pros and cons of the newest diet trend, 2019), and a meta-analysis of 17 randomised trials found «no clear evidence that intermittent fasting interventions affect hunger» compared with ordinary calorie cutting, with very low certainty (Elsworth et al., Nutrients, 2023).
The practical test is whether an 8-hour window makes you eat less without feeling deprived. If it does not after a few weeks, a different window, or a slower start, may fit better.
14:10, 16:8 or 18:6
16:8 sits in the middle of the daily plans. No trial we found compared 18:6 with 16:8 directly, and the one trial we found that compared 16:8 with 14:10 fasted three days a week.
| Plan | Typical window | What the trials show | Who tends to pick it |
|---|---|---|---|
| 14:10 | 9 am to 7 pm | One 12-week trial in 137 firefighters on 24-hour shifts (Manoogian 2022) | Breakfast eaters, late diners, shift workers |
| 16:8 | 12 pm to 8 pm | Small losses eating freely (Lowe, Gabel); on a calorie limit, nothing significant on top (Liu) or 2.3 kg more (Jamshed) | People who already skip breakfast |
| 18:6 | 1 pm to 7 pm | About 3% lost in 8 weeks eating freely; mild side effects peaked at week 2 (Cienfuegos 2020) | People settled on 16:8 who want a shorter window |
That part-time comparison ran over 3 months: 99 adults with obesity and type 2 diabetes in Thailand fasted on three days a week and lost 4.02% of body weight on 16:8, 3.15% on 14:10 and 0.55% in the control group (Sukkriang and Buranapin, Journal of Diabetes Investigation, 2024). It says nothing about a daily plan in people without diabetes.
For 20:4, one meal a day and fasts of 24 hours or longer, see how long a fast should be.
Who 16:8 suits and who it does not
16:8 tends to fit if:
- you already skip breakfast, or late-evening snacks are your weak point
- you would rather have one rule than count calories
- your main meal is in the evening, with other people
- your days have a steady shape, so the same 8 hours are free most days
The Cleveland Clinic adds a caveat: «If you’re very active or if you wake up hungry for breakfast, you may need to make some adjustments» (Cleveland Clinic, 2026).
It does not suit anyone whose health depends on regular meals. Harvard’s Nutrition Source says people with diabetes, an eating disorder that involves unhealthy self-restriction, medication that requires food intake, an active growth stage such as adolescence, or a pregnancy or breastfeeding should abstain (Harvard Nutrition Source, undated). Who should not do intermittent fasting has the other medical lists and what to ask your doctor.
Your first week
Build the gap over the week rather than starting at 16 hours on day 1. Harvard Health’s advice is that «it’s often a good idea to ease into the changes» (Harvard Health, 2026). The example uses the standard window; for an early window, keep the 8 am start fixed and move dinner earlier instead.
- Before day 1. Pick your window from the table above and tell the people you eat with. Stock water, carbonated water, black coffee and unsweetened tea. If you use Bare Fast, make 16:8 your usual plan now, with an 8 pm start and a daily reminder; on days 1 to 4 you end the fast early on purpose.
- Days 1 and 2: 12 hours. Finish dinner by 8 pm and eat again at 8 am. Most of the fast is sleep.
- Days 3 and 4: 14 hours. Same dinner, first meal at 10 am.
- Days 5 to 7: 16 hours. First meal at 12 pm. You are on 16:8.
- Week 2. Hold the window.
In the one trial cited here that tracked side effects week by week (a trial of 18:6 and 20:4, with no 16:8 arm), mild dizziness, nausea, headaches and diarrhoea peaked at week 2 (Cienfuegos et al., 2020). If a symptom does not fade, or you feel faint, end the fast that day and talk to your doctor before you continue.
Track it if you like, but do not let the count run you. In Bare Fast an early end reads «of 16 h», never «late» or «failed», so a 14-hour day in week one is a 14-hour day, not a failure.
Common questions
Does coffee with milk break a 16:8 fast?
By the Cleveland Clinic’s rule, yes: black coffee and unsweetened tea are fine, and anything with calories is not (Cleveland Clinic, 2026). No trial or medical page cited here sets a calorie threshold below which a fast still counts. If a splash of milk is what makes 16 hours keepable for you, that is a trade-off you make knowingly.
Can I take weekends off 16:8?
None of the daily trials cited here tested weekends off; each kept one window for the whole study. Since the window’s effect in trials came from eating less, a weekend that makes up the difference would cancel it. A practical rule: keep the window within an hour or two on weekends, and log a late Saturday as an early end. If you want that looser weekend built in, Bare Fast Pro’s weekly Schedule can set a shorter plan, such as 14:10, for Saturday and Sunday.
Can I exercise during the 16 fasting hours?
For steady cardio, a small trial found no difference: over 4 weeks, 20 young women on a calorie deficit did aerobic exercise fasted or fed, and body composition changes were «similar regardless whether or not an individual is fasted prior to training» (Schoenfeld et al., Journal of the International Society of Sports Nutrition, 2014). For lifting, the 16:8 trial in trained men put the sessions inside the window: over 8 weeks, 34 resistance-trained men trained between 4 pm and 6 pm and kept their fat-free mass and maximal strength (Moro et al., Journal of Translational Medicine, 2016). The Cleveland Clinic advises scheduling workouts when you are not fasting, because fasted exercise carries «a much higher risk of dehydration and heat illness», and to stop if you are «slowing down, getting bad reps or feeling clumsy» (Cleveland Clinic, 2024).
Does the 16:8 window have to be the same every day?
The trials kept it the same, and none tested a moving window. If you have to shift, move the whole window by an hour or two rather than one end of it: a 1 pm start with an 8 pm finish is 17:7.
How long until 16:8 feels normal?
No trial cited here measured it. Side effects were tracked week by week in only one trial cited here (18:6 and 20:4, with no 16:8 arm), where they peaked at week 2 (Cienfuegos 2020), and a meta-analysis of 17 trials found no clear evidence that fasting changes hunger compared with ordinary calorie cutting (Elsworth et al., 2023), so do not count on hunger disappearing.
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.