12-hour overnight fast

What it is

All eating happens within about 12 hours of the day, leaving roughly 12 hours without food overnight. In trials it is mostly used as the comparison schedule: a 12-hour feeding period was the control in a controlled-feeding trial of early time-restricted eating, and an eating window of 12 or more hours was the control in a 14-week weight-loss trial.

What the research says

There are almost no randomised trials testing a 12-hour window as the intervention. It served as the control arm in two early time-restricted eating trials, where it produced less improvement in insulin sensitivity (feeding trial in men with prediabetes) and less weight loss (by 2.3 kg over 14 weeks) than an 8-hour or 6-hour early window. In a cohort of 2,413 women with early breast cancer, nightly fasting under 13 hours was associated with higher recurrence risk (HR 1.36) but not with breast cancer or all-cause mortality. A systematic review of windows of 12 hours or less found no major change in disordered-eating questionnaires, but higher hunger at bedtime.

Evidence grade: Not enough research to say.

The schedule

• Pattern: daily
• Fasting hours: 12
• Eating hours: 12

Why people choose it

It is the gentlest schedule in this group and close to how many people already eat, so it is the usual comparison arm in trials rather than a treatment on its own. When shorter early windows were compared against it, the shorter windows did better on insulin sensitivity in a small feeding trial and on weight in a 14-week trial. The only long-term human data on overnight fasting length are observational: in women treated for breast cancer, fasting under 13 hours a night was associated with more recurrence but not with more deaths.

Cautions

• people with a history of eating disorders: The Cochrane team warned its fasting results cannot be applied to everyone and may differ with underlying eating disorders or behaviours. (Source 1)
• people who find hunger distressing: Interviews in time-restricted eating studies (windows of 12 hours or less) described fear of hunger and eating-related stress, and bedtime hunger was higher. (Source 2)
• people taking insulin or other glucose-lowering (antihyperglycaemic) medicines: A 2026 meta-analysis of time-restricted eating in type 2 diabetes said that for patients taking antihyperglycaemic agents, blood glucose should be closely monitored and medicines adjusted in good time, both to prevent hypoglycaemia and because moving meal times can disrupt usual dosing. It did not name specific drug classes in that passage. (Source 3)
• adults over 64: The main US app-based trial of daily time-restricted eating enrolled only adults aged 18 to 64 with a BMI of 27 to 43, so older and leaner adults were not studied. (Source 4)

What we did not find on harm

No trial was found that tested a 12-hour window as the intervention and reported adverse events; it appears only as a control arm. The harm listed comes from a systematic review that pools windows of 12 hours or less. (Source 2)

What goes wrong

A mixed-methods systematic review of 16 studies of eating windows of 12 hours or less found that disordered-eating questionnaires did not change, but hunger at bedtime was higher and interviews described fear of hunger and eating-related stress. (Source 2)

• Systematic review, Certainty not rated.
• Size: 16 studies.
• Who: adults in trials with a daily eating window of 12 hours or less.
• How long: varied.
• Result: Higher bedtime appetite; morning appetite lower or unchanged; questionnaires not different; qualitative themes of fear of hunger and eating-related stressors.
• Funding: not stated.

Limit of this finding: Pools windows of 12 hours and shorter; not specific to 12:12.

Qualitative themes converged with these findings, however also showed fear of hunger, eating in the absence of hunger, and eating-related stressors.

What the evidence supports

In a cohort of women treated for early-stage breast cancer, nightly fasting of less than 13 hours was associated with a higher risk of breast cancer recurrence than fasting 13 or more hours. (Source 5)

• Cohort study, Certainty not rated.
• Size: 2,413 women.
• Who: women aged 27 to 70 at diagnosis with early-stage breast cancer and without diabetes (Women's Healthy Eating and Living study)
• How long: mean 7.3 years follow-up for recurrence.
• Result: HR 1.36 (95% CI 1.05-1.76) for fasting <13 h vs >=13 h per night; mean nightly fast 12.5 (SD 1.7) hours.
• Funding: independent: NCI/NIH grants listed in the Europe PMC record.

Limit of this finding: Observational association from 24-hour dietary recalls; it does not show that lengthening the overnight fast changes recurrence. Supports a fast of at least 13 hours rather than 12.

In repeated-measures Cox proportional hazards regression models, fasting less than 13 hours per night (lower 2 tertiles of nightly fasting distribution) was associated with an increase in the risk of breast cancer recurrence compared with fasting 13 or more hours per night (hazard ratio, 1.36; 95% CI, 1.05-1.76).

In the same cohort, each 2-hour longer nightly fast was associated with slightly lower HbA1c and longer night-time sleep. (Source 5)

• Cohort study, Certainty not rated.
• Size: 2,413 women.
• Who: women with early-stage breast cancer without diabetes.
• How long: cross-sectional associations within the cohort.
• Result: HbA1c beta -0.37 (95% CI -0.72 to -0.01) per 2-hour increase; sleep beta 0.20 (95% CI 0.14-0.26)
• Funding: independent: NCI/NIH grants listed in the Europe PMC record.

In multivariable linear regression models, each 2-hour increase in the nightly fasting duration was associated with significantly lower hemoglobin A1c levels (β = -0.37; 95% CI, -0.72 to -0.01) and a longer duration of nighttime sleep (β = 0.20; 95% CI, 0.14-0.26).

What the evidence does not support

In a controlled-feeding crossover trial in men with prediabetes, the 12-hour feeding period was the control schedule, and a 6-hour early window improved insulin sensitivity, blood pressure, oxidative stress and appetite relative to it. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 8 men.
• Who: men with prediabetes, fed enough to maintain weight.
• How long: 5 weeks per schedule, crossover.
• Result: eTRF improved insulin sensitivity, beta cell responsiveness, blood pressure, oxidative stress and appetite versus the 12-hour schedule; no numbers in the recorded abstract.
• Funding: independent (NIGMS, NCATS, NORC grants)

Limit of this finding: Small proof-of-concept trial in men only; the 12-hour arm was a comparison, not the intervention being promoted.

Men with prediabetes were randomized to eTRF (6-hr feeding period, with dinner before 3 p.m.) or a control schedule (12-hr feeding period) for 5 weeks and later crossed over to the other schedule. eTRF improved insulin sensitivity, β cell responsiveness, blood pressure, oxidative stress, and appetite.

In a 14-week randomised trial with matched weight-loss counselling, people eating over 12 or more hours a day lost 2.3 kg less weight than people eating in an 8-hour early window, but the difference in body fat was not significant. (Source 7)

• Randomized trial, Certainty not rated.
• Size: 90 adults enrolled.
• Who: adults aged 25 to 75 with obesity (mean BMI 39.6; 80% female) in a hospital weight-loss clinic.
• How long: 14 weeks.
• Result: Weight -2.3 kg (95% CI -3.7 to -0.9; P = .002) favouring eTRE+ER over the >=12-hour control; body fat -1.4 kg (95% CI -2.9 to 0.2; P = .09)
• Funding: independent: NIH grants P30 DK056336, T32 HL105349 and P30 DK072476, as listed in the Europe PMC grant record.

Limit of this finding: Both arms also received energy restriction; the 12-hour arm was the control.

In this randomized clinical trial, eTRE was more effective for losing weight and improving diastolic blood pressure and mood than eating over a window of 12 or more hours at 14 weeks.

In the same cohort, nightly fasting of less than 13 hours was not associated with a statistically significant higher risk of breast cancer death or overall mortality. (Source 5)

• Cohort study, Certainty not rated.
• Size: 2,413 women.
• Who: women with early-stage breast cancer without diabetes.
• How long: mean 11.4 years of surveillance for mortality.
• Result: Breast cancer mortality HR 1.21 (95% CI 0.91-1.60); all-cause mortality HR 1.22 (95% CI 0.95-1.56)
• Funding: independent: NCI/NIH grants listed in the Europe PMC record.

Nightly fasting less than 13 hours was not associated with a statistically significant higher risk of breast cancer mortality (hazard ratio, 1.21; 95% CI, 0.91-1.60) or a statistically significant higher risk of all-cause mortality (hazard ratio, 1.22; 95% CI, 0.95-1.56).

Where the evidence is mixed

In an unreviewed conference abstract using US national survey data, eating across less than 8 hours a day was not associated with living longer than a standard 12 to 16 hour eating day. (Source 8)

• Cohort study, Certainty not rated.
• Size: about 20,000 US adults, average age 49.
• Who: NHANES 2003-2018 participants linked to the National Death Index.
• How long: deaths recorded through December 2019.
• Result: No survival advantage for <8-hour eating compared with the 12-16 hour reference group.
• Funding: not stated.

Limit of this finding: Conference abstract, not peer reviewed; eating window estimated from two days of recall in a cross-sectional survey later linked to death records. Here the 12 to 16 hour group is the reference, not a tested intervention.

Compared with a standard schedule of eating across 12-16 hours per day, limiting food intake to less than 8 hours per day was not associated with living longer.

A common belief, and what the research shows

The belief: A 12-hour overnight fast is a proven form of intermittent fasting with its own health benefits.

What the research shows: It has mostly been used as the comparison arm. Shorter early windows beat it on insulin sensitivity and weight in small trials, and the only long-term human data on overnight fasting length are observational.

References

1. Cochrane (news release). Evidence behind intermittent fasting for weight loss fails to match hype. 2026. Read the source
2. Appetite. The impact of time restricted eating on appetite and disordered eating in adults: A mixed methods systematic review. 2023. DOI 10.1016/j.appet.2023.106452. Read the source
3. Diabetology & Metabolic Syndrome. Effects of time-restricted eating on people with type 2 diabetes: a systematic review and meta-analysis. 2026. DOI 10.1186/s13098-026-02138-8. Read the source
4. JAMA Internal Medicine. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. 2020. PMID 32986097, DOI 10.1001/jamainternmed.2020.4153. Read the source
5. JAMA Oncology. Prolonged Nightly Fasting and Breast Cancer Prognosis. 2016. PMID 27032109, DOI 10.1001/jamaoncol.2016.0164. 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. JAMA Internal Medicine. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. 2022. PMID 35939311, DOI 10.1001/jamainternmed.2022.3050. Read the source
8. EurekAlert! / American Heart Association. 8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death (EurekAlert copy of the AHA news release). 2024. Read the source