18:6 time-restricted eating

What it is

All food is eaten within a 6-hour window each day, and nothing is eaten for the remaining 18 hours. In the main weight-loss trial the window ran from 1 to 7 p.m. with no calorie counting. In the early version tested in men with prediabetes, the window ended before 3 p.m.

What the research says

Evidence is limited to short trials, 5 to 8 weeks long, with small numbers. One randomised trial in adults with obesity found about 3% weight loss over 8 weeks versus controls, with lower insulin resistance and oxidative stress. A tiny crossover trial fed men enough food to hold their weight steady and found better insulin sensitivity and blood pressure on an early 6-hour window. A network meta-analysis of 41 trials found that window length (<8 h, 8 h, >8 h) had inconsistent effects. The time of day of eating (early rather than late) mattered more. Lean mass fell in the 6-hour arm, and a non-peer-reviewed cohort abstract linked eating windows under 8 hours with higher cardiovascular death.

Evidence grade: Limited evidence.

The schedule

• Pattern: daily
• Fasting hours: 18
• Eating hours: 6

Why people choose it

It is a daily rule with no calorie counting. In an 8-week trial it cut intake by about 550 kcal a day and weight by about 3% compared with no timing limit. Nearly all men in a small supervised trial called an 18-hour daily fast not difficult or only moderately difficult. It has not been shown to beat a wider 8-hour window or ordinary calorie restriction on weight: a network meta-analysis found no consistent effect of window length. It also lost more lean mass than the 4-hour window in one trial.

Cautions

• People taking insulin or sulfonylureas: Hypoglycaemia is the immediate risk for people with diabetes who take these medicines. Trials in type 2 diabetes did not see more hypoglycaemia when glucose was monitored and medicines were adjusted to protocol. (Source 1)
• People taking insulin or sulfonylureas (trial conditions): The absence of extra hypoglycaemia in trials depended on regular glucose monitoring and medicine adjustment written into the protocol. (Source 2)
• Pregnant or breastfeeding women, frail older adults, children, people with eating disorders: Johns Hopkins patient guidance lists these groups as people who should not follow intermittent fasting. (Source 1)
• Older adults (muscle loss): Some trials of time-restricted eating reported clinically significant lean mass loss in older participants, possibly because they ate too little protein. (Source 2)
• People taking blood pressure medicines: The factsheet says antihypertensives may need adjusting because of the blood pressure-lowering effect of low-calorie days and subsequent weight loss. That refers to intermittent energy restriction (low-calorie days); the factsheet does not say it about daily time-restricted eating. (Source 2)
• People with a history of eating disorders: The factsheet notes concern that intermittent fasting could trigger eating disorders, but adds in the next sentence that studies of alternate-day fasting in people with overweight or obesity showed reductions or no change in eating disorders, and that no change was reported with time-restricted eating. (Source 2)

What goes wrong

Over 8 weeks, the 6-hour group lost significantly more lean mass than both the 4-hour group and controls (-1.5 kg versus -0.8 kg and -0.3 kg). (Source 3)

• Randomized trial, Certainty not rated.
• Size: 58 randomised; 49 completers.
• Who: Adults with obesity.
• How long: 8 weeks.
• Result: Lean mass change -1.5 +/- 0.2 kg (6-h) vs -0.8 +/- 0.4 kg (4-h), p = 0.04, and vs -0.3 +/- 0.2 kg (controls), p = 0.03.
• Funding: independent (NIH)

Lean mass by week 8 was significantly different across the three groups (p = 0.04), with the 6-h TRF group (Δ = −1.5 ± 0.2 kg) losing significantly more lean mass than the 4-h TRF group (Δ = −0.8 ± 0.4 kg, p = 0.04) and controls (Δ = −0.3 ± 0.2 kg, p = 0.03), with no difference between the 4-h TRF group and controls (Figure 4B).

Mild adverse events, including dizziness, nausea, headaches and diarrhoea, peaked at week 2 of 4-h and 6-h eating. No serious adverse events were reported. (Source 3)

• Randomized trial, Certainty not rated.
• Size: 58 randomised; 49 completers.
• Who: Adults with obesity.
• How long: 8 weeks.
• Result: Dizziness, nausea, headaches and diarrhoea peaked at week 2 and disappeared by week 3. Constipation and dry mouth, seen in both TRF groups at week 2, persisted through some or most of the study (see the separate finding).
• Funding: independent (NIH)

No serious adverse events were reported. Mild adverse events, such as dizziness, nausea, headaches, and diarrhea, peaked at week 2 in both TRF interventions, relative to controls, but disappeared by week 3 and did not reoccur during the trial.

In the same trial, constipation and dry mouth appeared at week 2 in both the 4-hour and 6-hour groups and persisted through some or most of the study. (Source 3)

• Randomized trial, Certainty not rated.
• Size: 58 randomised; 49 completers.
• Who: Adults with obesity.
• How long: 8 weeks.
• Result: Constipation and dry mouth relative to controls, persisting through some or most of the 8-week intervention; no numbers given in the text read.
• Funding: independent (NIH)

were observed in both the 4- and 6-h TRF groups at week 2 relative to controls, and these issues persisted throughout some or most of the study.

Most types of time-restricted eating reduced lean mass (fat-free mass) compared with usual diets; compared with continuous energy restriction, the difference in lean mass was not statistically significant. (Source 4)

• Meta-analysis, Certainty not rated.
• Size: 41 randomised controlled trials, 2,287 participants.
• Who: Adults in time-restricted eating trials.
• How long: Median 8 weeks.
• Result: Significant reduction in fat free mass-lean mass versus usual diets for most TRE types. No significant difference versus continuous energy restriction (not a test of equivalence).
• Funding: independent.

Most time restricted eating types significantly reduced fat free mass-lean mass compared with usual diets but no significant difference was seen compared with continuous energy restriction.

In a US survey cohort, eating all food within less than 8 hours a day was associated with a 91% higher risk of cardiovascular death. (Source 5)

• Cohort study, Certainty not rated.
• Size: About 20,000 US adults.
• Who: US adults (NHANES dietary recall linked to death records)
• How long: Median follow-up 8 years, maximum 17 years.
• Result: 91% higher risk of cardiovascular death for an eating duration under 8 hours. Not associated with lower all-cause death.
• Funding: not stated.

Limit of this finding: This is a conference abstract (AHA EPI|Lifestyle 2024), not peer reviewed and described by AHA as preliminary. Eating windows came from self-reported 24-hour recall, not from people deliberately following 18:6. It was read via the AHA newsroom release.

People who followed a pattern of eating all of their food across less than 8 hours per day had a 91% higher risk of death due to cardiovascular disease.

What the evidence supports

In adults with obesity, eating only between 1 and 7 p.m. for 8 weeks reduced body weight by about 3% versus a no-restriction control, without calorie counting. (Source 3)

• Randomized trial, Certainty not rated.
• Size: Adults with obesity randomised to 4-h TRF, 6-h TRF or control; 19 completers in the 6-h group, 49 completers in total.
• Who: Adults with obesity (Chicago, NCT03867773)
• How long: 8 weeks of intervention (10-week trial)
• Result: About 3% weight loss versus controls. Energy intake down about 550 kcal/day. Insulin resistance and oxidative stress fell versus controls.
• Funding: independent (NIH R01DK119783)

Limit of this finding: Small, short trial compared with a no-intervention control, not with calorie restriction.

After 8 weeks, 4- and 6-h TRF produced comparable reductions in body weight (∼3%), insulin resistance, and oxidative stress, versus controls.

Men with prediabetes who ate within an early 6-hour window, ending before 3 p.m., and were fed enough to keep their weight stable had better insulin sensitivity, blood pressure, oxidative stress and appetite than on a 12-hour schedule. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 8 men completed (randomised crossover)
• Who: Overweight men with prediabetes, mean age 56 +/- 9 years.
• How long: 5 weeks per arm.
• Result: Improvements in insulin sensitivity, beta cell responsiveness, blood pressure, oxidative stress and appetite, without weight loss.
• Funding: independent (NIGMS, NCATS, NORC grants)

Limit of this finding: Only 8 men completed, all food was supplied, and the window was early in the day. The results may not apply to a 1 to 7 p.m. window.

eTRF improved insulin sensitivity, β cell responsiveness, blood pressure, oxidative stress, and appetite.

In the early 6-hour trial, all but one participant rated fasting for 18 hours a day as not difficult or only moderately difficult. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 8 men.
• Who: Overweight men with prediabetes.
• How long: 5 weeks.
• Result: 7 of 8 rated difficulty below 50 mm on a 100-mm scale.
• Funding: independent (NIGMS, NCATS, NORC grants)

Limit of this finding: Supervised feeding in a very small sample. The same participants thought a window of about 7.8 hours would be feasible for most people. The same participants rated eating within the 6-hour window as harder than the 18-hour fast.

In fact, all but one participant reported that it was not difficult or only moderately difficult (<50 mm on a 100-mm scale) to fast for 18 hr daily.

What the evidence does not support

Across 41 randomised trials, eating windows shorter than 8 hours did not show a consistent advantage over 8-hour or longer windows. The time of day mattered more than window length. (Source 4)

• Meta-analysis, Moderate certainty.
• Size: 41 randomised controlled trials, 2,287 participants.
• Who: Adults in time-restricted eating trials longer than one month.
• How long: Median 8 weeks.
• Result: P value rankings for eating duration were inconsistent. A <8 h window lowered fasting insulin by 1.48 uIU/ml (95% CI -2.90 to -0.05; moderate certainty) versus an 8 h window.
• Funding: independent (Taiwan Ministry of Science and Technology; National Taiwan University)

Limit of this finding: About 60.2% of the network evidence was moderate to high certainty. The authors say direct evidence was limited for late time-restricted eating with an eating duration of under 8 hours.

The association between duration of eating and metabolic health outcomes was inconsistent.

Sleep quality, sleep duration, insomnia severity and sleep apnoea symptoms did not change with 6-hour eating compared with controls. (Source 7)

• Randomized trial, Certainty not rated.
• Size: 49 adults with obesity across 4-h, 6-h and control groups.
• Who: Adults with obesity.
• How long: 8 weeks.
• Result: PSQI 6.4 to 5.3 in the 6-h group, not different from controls.
• Funding: not stated in the abstract read (parent trial NIH-funded)

These findings suggest that 4- and 6-h TRF have no effect on sleep quality, duration, insomnia severity, or the risk of obstructive sleep apnea.

Where the evidence is mixed

Compared with continuous energy restriction, early time-restricted eating reduced body weight by about 1.1 kg more. This comparison was by time of day, not by 6-hour windows specifically. (Source 4)

• Meta-analysis, High certainty.
• Size: 41 randomised controlled trials, 2,287 participants (network)
• Who: Adults in time-restricted eating trials.
• How long: Median 8 weeks.
• Result: -1.13 kg (95% CI -1.92 to -0.35), high certainty, for early TRE versus continuous energy restriction.
• Funding: independent.

Limit of this finding: There is no direct trial of an 18:6 window against daily calorie restriction. The benefit is tied to eating early in the day.

Compared with continuous energy restriction, early time restricted eating significantly reduced body weight (−1.13 kg, 95% CI −1.92 to −0.35; high certainty of evidence)

Where the research disagrees

Whether windows shorter than 8 hours carry extra risk or extra benefit

• Network meta-analysis of 41 RCTs (BMJ Medicine 2026), meta-analysis: Window length had inconsistent effects on metabolic outcomes. A <8 h window modestly lowered fasting insulin compared with 8 h, and no serious adverse events were reported in the 14 trials with data. (Source 4)
• AHA EPI|Lifestyle 2024 conference abstract (NHANES), cohort: Self-reported eating within less than 8 hours a day was associated with higher cardiovascular death. The abstract is preliminary and was not peer reviewed. (Source 5)

A common belief, and what the research shows

The belief: A shorter eating window, such as 6 hours rather than 8, gives more weight loss and better metabolic health.

What the research shows: Across 41 trials, window length had inconsistent effects. Eating earlier in the day did better than eating later. The 6-hour arm of one trial lost no more weight than the 4-hour arm but lost more lean mass.

References

1. Johns Hopkins Medicine (patient guidance; position). An Overview of Intermittent Fasting (The Johns Hopkins Patient Guide to Diabetes). 2020 (modified 2024). Read the source
2. Diabetes & Primary Care 26(3) (Harvie M, McDiarmid S). At a glance factsheet: Intermittent fasting for the management of weight and diabetes. 2024. Read the source
3. Cell Metabolism. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. 2020. PMID 32673591, DOI 10.1016/j.cmet.2020.06.018. Read the source
4. BMJ Medicine. Effects of timing and eating duration of time restricted eating on metabolic outcomes: systematic review and network meta-analysis. 2026. DOI 10.1136/bmjmed-2024-001071. Read the source
5. American Heart Association (newsroom); abstract in Circulation 2024;149(Suppl 1):P192. 8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death (AHA newsroom release on EPI|Lifestyle 2024 abstract P192, 'Association Between Time-Restricted Eating and All-Cause and Cause-Specific Mortality'). 2024. DOI 10.1161/circ.149.suppl_1.P192. Read the source
6. Cell Metabolism. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. 2018. PMID 29754952, DOI 10.1016/j.cmet.2018.04.010. Read the source
7. Nutrition and Health (SAGE). The effect of 4-h versus 6-h time restricted feeding on sleep quality, duration, insomnia severity and obstructive sleep apnea in adults with obesity. 2022. PMID 33759620, DOI 10.1177/02601060211002347. Read the source