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Time-restricted6 studies in people cited

18:6 Time-Restricted Eating

18 hours without food · Daily

Written by Reviewed Sep 2026

Also called: 6-hour eating window, 18-hour fast

You eat inside 6 hours and fast for 18. In a study that put it head to head with a 4-hour window, both lost the same weight. But on the 6-hour window, people lost slightly more lean mass than fat.

Overview

18:6 squeezes your eating into six hours. It has been tested three ways. Against a 4-hour window in adults with obesity. In women with polycystic ovary syndrome (PCOS) over six months. And in men with prediabetes who were fed every meal, with the window early in the day and their weight kept steady.

The most useful study for anyone thinking about it is Cienfuegos 2020. It ran a 6-hour window and a 4-hour window side by side, with the same comparison group. Both lost the same amount of weight. They did not lose the same kind of tissue.

In both short-window groups, people ate about 550 fewer calories a day without counting anything. That is the biggest drop in calories that happened on its own in any eating window study. It is the most likely reason they lost weight.

How it works

Cienfuegos used set windows: 1pm to 7pm (13:00 to 19:00) for the 6-hour group and 3pm to 7pm (15:00 to 19:00) for the 4-hour group. The people had a BMI of 30 to 50. There were no rules about what they ate in the window. Corapi used 1pm to 7pm for six months in women with PCOS, again with no calorie tracking. Sutton put the six hours early, with dinner done before 3pm (15:00), and gave people every meal.

In Cienfuegos, how well people stuck with it did not change over the eight weeks. That is worth knowing, because you might expect a short window to fall apart faster.

The real problem with this window is math. If you aim for 1.6 g of protein per kg of body weight a day, you have six hours to eat it. That usually means three big protein meals, not the four to six that most protein research is built around.

What 18 hours actually reaches

An 18-hour daily fast has been shown to lower blood sugar across the day and lower a marker of wear and tear on cells (8-isoprostane). In Cienfuegos, that marker fell 12 pg/mL in the 6-hour group and 13 pg/mL in the 4-hour group, a 37% drop. Fasting insulin and insulin resistance improved in both groups. Fasting blood sugar did not clearly change. The authors noted this was the only marker of that kind of cell wear they measured. This is also the window length where the one direct measurement of autophagy (the cell's clean-up process) in an eating window study sits. In 11 overweight adults, each person tried an 8am to 2pm window (08:00 to 14:00) and an 8am to 8pm window (08:00 to 20:00) for four days each. The early six-hour window turned up the clean-up gene LC3A and a gene called SIRT1 in blood cells before breakfast. It also turned up a gene called MTOR in the evening. Average blood sugar across 24 hours fell 4 mg/dL. That is gene activity in blood cells over four days. It is not the clean-up process itself, and it is not in muscle or liver. And the window was early in the day. It is the reason this page lists autophagy as something this window reaches. It does not mean there is a set hour when autophagy switches on.

Fig. 1 · what has been measured, hour by hour
  1. 0h to 4h

    Still digesting

    Burning
    Sugar from the meal you just ate. Insulin is high enough to keep fat locked away.
    • Insulin is at its after-meal peak. While it is up there, the enzyme that releases fat from fat cells stays switched off, so fat is being stored, not released.
    • The hour count on most fasting charts starts at the last bite. But the body is still digesting a mixed meal here, so the clock and what the body is doing are not yet the same thing.

    Read the biologyInsulin and insulin sensitivity during a fast

    Measured in people
    The hour-by-hour fasting studies in people start later than this. Klein 1993 took its first sample at 12 hours, in 6 healthy men. Rothman 1991 gives its first figure for new sugar making as an average across the first 22 hours. This row is the fed state those measurements are compared against. No measurement from a long fast falls inside it.

  2. 4h to 12h

    Meal digested, overnight fast

    Burning
    A mix of leftover sugar and fat. The liver keeps the brain supplied with sugar.
    Blood sugar
    5.58 mmol/L (100.5 mg/dL) at the 12 hour mark, in 6 healthy men
    Ketones
    Total ketones around 0.20 mmol/L after an overnight fast. That figure is acetoacetate and BHB added together, not BHB on its own.
    Insulin
    64.6 pmol/L (9.3 microunits/mL) at 12 hours, in the same 6 men
    • At 12 hours, glycerol is released at 2.08 and palmitic acid at 1.63 micromol/kg/min. These are two markers of fat being released. They are the starting numbers everything later in the fast is compared against.
    • Making new sugar from scratch already supplies most of the body's sugar at this point. It is not a backup waiting to be called on. This is the first place the popular "switch" story breaks down.

    Read the biologyInsulin and insulin sensitivity during a fastGlycogen depletion and gluconeogenesis

    Measured in people
    Klein 1993 measured blood sugar, insulin and fat release with tracers at 12 hours, in 6 healthy men. Féry 1983 measured total ketones in people after an overnight fast and put the starting level at 0.20 mmol/L. Rothman 1991 measured liver glycogen again and again with a special MRI scan through a 68 hour fast in healthy adults. It worked out that new sugar making supplied 64% of all the body's sugar across the first 22 hours.

  3. 12h to 18h

    Ketones have not moved yet

    Burning
    Fat use rising. More and more of the body's sugar is being made fresh, not pulled from storage.
    Ketones
    A middle change of 0% between 12 and 18 hours, measured every 6 hours in 34 adults
    • BHB, the main ketone, does not move in this window. In 34 adults sampled every 6 hours through the standard 72 hour fast doctors use for testing, the middle change from 12 to 18 hours was zero.
    • Making new sugar supplies 64% of the body's sugar across the first 22 hours. A second, unrelated method put it at about 47% by 14 hours. The body does not switch from stored sugar to fat at a set hour.
    • In 11 overweight adults, activity of the autophagy gene LC3A in blood cells was 22% higher after an 18 hour fast than after a 12 hour fast. In the same people, activity of the MTOR gene also rose, by 9%.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Service 2005 measured BHB every 6 hours in 34 adults whose 72 hour test fasts came back clear for a rare insulin-making tumour. Rothman 1991 measured the share of new sugar making with a special MRI scan. Jamshed 2019 measured gene activity in blood cells in an 11-person, 4 day study where each person tried two eating windows: 8am to 2pm, and 8am to 8pm. That is one gene's activity in blood cells. It is not the real speed of autophagy, and it is not muscle. No study in any species has tested a 16 hour time point for autophagy.

  4. 18h to 1d

    The biggest hours for fat release

    past this window
    Burning
    Fat and glycerol, with ketone making starting its climb
    Ketones
    Climbing. The middle rise from 18 to 36 hours was +333%, the steepest stretch of a 72 hour fast.
    Insulin
    70% of the whole drop seen across a 72 hour fast has already happened by 24 hours
    • The single biggest jump in fat release across a whole 72 hour fast comes between 18 and 24 hours. 60% of the total rise in fat release happens between 12 and 24 hours. The authors put that down to falling insulin, not to any change in blood sugar.
    • In a study of 30 healthy adults, growth hormone was clearly higher after a 24 hour water-only fast than after a normal eating day. Each person did both. The size of that rise is not in the published summary, so it is not given here.
    • In the same study, haemoglobin, red blood cell count and haematocrit all rose together at 24 hours. That pattern means the fluid part of the blood shrank. It does not mean new red blood cells were made.
    • In 20 volunteers, energy burned over 24 hours, measured in a sealed room, fell during a 24 hour fast. How much it fell varied from person to person.

    Read the biologyInsulin and insulin sensitivity during a fastThe growth hormone responseKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Horne 2013 had 30 seemingly healthy adults do both a 24 hour water-only fast and a normal eating day, in random order. It measured growth hormone, blood counts, cholesterol, triglycerides, bicarbonate and weight. Klein 1993 measured fat release with tracers. Hollstein 2020 measured energy burned over 24 hours in a sealed room in 20 volunteers. The exact percentage drop is not in the published summary, so it is not given here. On autophagy, a 24 hour fast raised p62 (also called SQSTM1) in the thigh muscle of 50 women. That points toward less recycling in muscle, not more.

  5. 1d to 36h

    Thyroid drops before metabolic rate does

    past this window
    Burning
    Fat. The remaining sugar is built from glycerol, lactate and protein building blocks.
    Ketones
    1.308 mmol/L on average after day 1, in 13 men starting a 10 day water fast, up from 0.177 at the start. People varied a lot: the spread around that average (1.053) was nearly as big as the average itself.
    • Making new sugar supplies 82% of the body's sugar between 22 and 36 hours.
    • In 8 healthy men, the thyroid hormone T3 fell from 2.30 to 1.84 nmol/L by 30 hours. TSH, the hormone that tells the thyroid to work, fell below 1 mU/L, and its usual night-time peak disappeared. Blood cortisol did not change.
    • So the thyroid slows down within the first 30 hours, well before anyone can measure a drop in resting metabolic rate.
    • Salt loss has not turned yet. In 9 women with obesity who kept eating their usual amount of salt while fasting, salt in the urine started to rise after about 48 hours and then went above what they took in.

    Read the biologyGlycogen depletion and gluconeogenesisKetogenesis and beta-hydroxybutyrateAutophagy

    Measured in people
    Rothman 1991 measured the share of new sugar making with a special MRI scan in healthy adults. Hugues 1984 fasted 8 healthy men for 30 hours and tested their thyroid hormones again and again. Dai 2022 measured blood BHB daily in 13 men fasting in a supervised facility. Sigler 1975 collected urine in 3 hour blocks from 9 women with obesity on a hospital ward. On autophagy, the only study in people with several time points in one fast took thigh muscle samples at 2, 12, 24 and 36 hours. The markers LC3I, LC3II and p62 all fell, but only in untrained people. The authors sum it up as muscle recycling being only mildly affected by 36 hours of fasting.

What studies in people found

There is one study that put 6-hour and 4-hour windows head to head, one six-month study in women with PCOS, and one study where people were fed every meal, the window was early and their weight was kept steady.

  • Corapi 2026, 76 women with PCOS for 6 months. They ate from 1pm to 7pm with no calorie tracking, or cut calories by 25% a day, or made no change. The window group lost 4.32% more weight than the no-change group. The calorie group lost 4.66%. The gap between the window and calorie cutting, 0.34%, was too small to count. There were no serious side effects.
  • Cienfuegos 2020, 58 adults for 8 weeks, split into 6-hour, 4-hour and no-change groups. Both fasting groups lost 3.2% of their body weight. The no-change group lost 0.1%. The two fasting groups did not differ. Fat fell 1.4 kg in the 6-hour group and 2.8 kg in the 4-hour group. Both groups ate about 550 fewer calories a day without counting. Belly fat did not differ between groups.
  • The autophagy study, 11 overweight adults. Each tried an 8am to 2pm window and an 8am to 8pm window for 4 days each. The early six-hour window turned up the genes LC3A and SIRT1 in blood cells before breakfast, and MTOR in the evening. It lowered average blood sugar across 24 hours by 4 mg/dL and blood sugar spikes by 12 mg/dL.
  • Sutton 2018, men with prediabetes, 8 of whom finished out of 130 screened. Each man tried both plans for 5 weeks. They were fed every meal and ate the same calories either way, so their weight stayed the same. The 6-hour window was early, with dinner done before 3pm. Everyone ate all the meals they were given. Insulin resistance improved and the body's insulin response to food got stronger. Morning blood pressure was 11 mmHg lower on top and 10 mmHg lower on the bottom. All of that happened without weight loss. Blood sugar did not improve, and the authors say so. With only 8 people, treat this as a first test of the idea.

What this does not tell you: Cienfuegos ran 8 weeks with 16 to 19 people finishing in each group. Sutton had 8 people finish. It also had a carry-over problem. After a 7-week break, all but one man who did the early window first started the second plan with insulin after meals at least 25% below where he began. The autophagy study measured gene activity in blood cells over four days. That is not the same as the clean-up process itself, and it is not muscle or liver. None of these studies measured protein per kg of body weight.

Reading the research record

The most interesting disagreement here is not between research groups. It is inside one study. In Cienfuegos, the 6-hour and 4-hour groups lost the same weight but lost it very differently. The 4-hour group lost twice the fat and half the lean mass. If the shorter window were just a harder version of the longer one, you would not expect that.

The second disagreement is about what studies can show when people are fed every meal versus living at home. Sutton fed people to keep their weight steady. It still measured better insulin handling, a stronger insulin response from the pancreas, and lower blood pressure. That is the best evidence so far that something happens beyond eating less. But it was 8 people, there was a known carry-over effect, and the window was early. So its results do not carry over neatly to the 1pm to 7pm window most people use.

On the other side, Templeman 2021 built a study to separate fasting from simply eating less. In 36 lean adults over 3 weeks, fasting did nothing extra for blood sugar control or heart health once calories were the same. Both results stand, and this page does not pick between them.

What happens to muscle

The 6-hour group in Cienfuegos 2020 gives the single most useful body make-up result at this window. It is worth reading twice. Lean mass means the non-fat part of you, mostly muscle and water. That group lost 1.5 kg of lean mass. That was more than the 4-hour group, which lost 0.8 kg, and more than the no-change group, which lost 0.3 kg. They lost 1.4 kg of fat. So they lost slightly more lean mass than fat. In the 4-hour group it went the other way: 2.8 kg of fat and 0.8 kg of lean.

That is at the bad end of the range across all the studies. The range runs from about 9% of weight lost as lean mass in TIMET, on a 10-hour window, to about 65% in TREAT, on an 8-hour window. The 6-hour group here was above both. Neither Cienfuegos group reported protein per kg of body weight, and neither lifted weights.

The protein research tells a steady, specific story about that spread. Short-window studies that fed 1.6 to 2.2 g of protein per kg a day and had people lift weights with supervision kept lean mass. Tinsley 2019 used 1.6. Stratton 2020 used 1.83, with both groups cutting calories by the same 25%. Moro 2016 used 1.93. Parr 2023, at 1.0 g per kg a day, lost more lean mass than its 12-hour comparison group, even though daily muscle building measured the same in both: 1.28 against 1.26 %/day.

What tracks lean mass is protein and lifting, not the number of hours. A six-hour window makes it harder to hit your protein target. That is a different problem from the window itself causing the loss.

Doing it

Protein is the constraint
Six hours means about three protein meals. In young men, 100 g of protein at once gave a bigger and longer muscle-building response than 25 g. It lasted more than 12 hours, and very little of the extra was burned off. So big servings are not wasted. The problem is getting enough in total, not a limit per meal.
Lift weights if lean mass matters
The 6-hour group in Cienfuegos lost more lean mass than fat, and they did not lift weights. Every short-window study that kept lean mass had people lift weights with supervision.
Window placement changes what was measured
The insulin and blood pressure results at this window come from an early window, with dinner before 3pm. The autophagy gene result comes from 8am to 2pm. The common 1pm to 7pm window has weight and PCOS results behind it, not those measurements.
What breaks the fast
The studies policed calories, not what you drank. No study at this window tested whether a tiny amount of calories is harmless.
Shorter is not automatically more
The 4-hour and 6-hour groups lost the same weight. In that one 8-week study, with 16 to 19 people finishing per group, going shorter than six hours changed which tissue was lost, not how much weight.

Breaking the fast

No study at this window set a first meal. The fed-every-meal study that came closest matched every meal to its comparison group. Here is the useful science. When you have not eaten for a while, muscle building ticks along at about 0.03 to 0.05 %/h. It rises when you eat, and that rise takes at least 90 minutes to start. In young men at rest, 15 g of essential amino acids (protein building blocks) gave the same muscle-building response whether it came in one serving or four. Getting your total in matters more than the order.

Sources

  1. Study in people2020
    Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity

    Cell Metabolism

    58 adults with obesity spent 8 weeks eating in a 4-hour window, a 6-hour window, or as usual. Both window groups lost 3.2% of body weight against 0.1%, and ate about 550 fewer calories a day without counting. The 6-hour group lost more lean mass, 1.5 kg, than the 4-hour group at 0.8 kg. A marker of cell damage fell in the window groups, and belly fat did not differ.

    58 randomised, 8 weeks, adults with obesity. 4-hour and 6-hour arms both lost 3.2% of body weight against 0.1% in control (P < 0.001), with no difference between them. Fat mass -1.4 kg (6-hour) and -2.8 kg (4-hour). Lean mass -1.5 kg in the 6-hour arm, more than the 4-hour arm at -0.8 kg (P = 0.04) and control at -0.3 kg (P = 0.03). Energy intake fell about 550 kcal/d without counting. 8-isoprostane fell 12 to 13 pg/mL across arms (P = 0.02). Visceral fat did not differ (P = 0.43).

  2. Study in people2026
    Time-restricted eating for body weight management in women with polycystic ovary syndrome: a randomized controlled trial

    Nature Medicine

    76 women with PCOS (polycystic ovary syndrome) spent 6 months on a 6-hour window from 13:00 to 19:00 with no calorie tracking, a 25% daily calorie cut, or no change. The window cut weight 4.32% more than no change, and the calorie cut did about the same at 4.66%. The gap between those two was too small to count as a real difference. There were no serious side effects, and period results were not in the summary.

    76 women with PCOS randomised to a 6-hour window (13:00 to 19:00, no calorie tracking), 25% daily calorie restriction, or control for 6 months. Weight fell 4.32% versus control (95% CI -6.20 to -2.44; P < 0.01) and 4.66% on calorie restriction, with no difference between them (0.34%; 95% CI -2.15 to 2.83; P = 0.79). No serious adverse events. Menstrual cycle outcomes are not in the abstract record.

  3. Study in people2018
    Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes

    Cell Metabolism

    Men with prediabetes each spent 5 weeks eating in a 6-hour window ending before 15:00 and 5 weeks eating over 12 hours, on the same supplied food. Only 8 finished, out of 130 screened. The early window improved the insulin response and cut blood pressure by 11 (top) and 10 (bottom) mmHg, with no weight change. Blood sugar itself did not improve, and some effects carried over between the two phases.

    5-week randomised crossover, isocaloric supervised feeding, 6-hour window with dinner before 15:00 versus 12 hours, in men with prediabetes. 8 completers screened from 130. Insulin resistance by 3-hour incremental AUC ratio -36 U/mg (P = 0.005), insulinogenic index +14 U/mg (P = 0.05), morning systolic -11 mmHg and diastolic -10 mmHg (both P = 0.03), 8-isoprostane -11 pg/mL (P = 0.05) partly driven by worsening in the control arm. Glucose did not improve. No weight change by design. Carryover was observed after a 7-week washout.

  4. Study in people2019
    Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans

    Nutrients

    11 adults with overweight each spent 4 days eating from 08:00 to 14:00 and 4 days eating from 08:00 to 20:00. The early six-hour window lowered average blood sugar over 24 hours by 4 mg/dL and cut swings by 12 mg/dL. It raised the activity of a cell clean-up (autophagy) gene and an ageing-related gene in blood cells. That is gene activity in blood over four days, not proof of clean-up in muscle or liver.

    11 overweight adults, 4-day randomised crossover of an 08:00 to 14:00 window versus 08:00 to 20:00. The early six-hour window raised whole blood cell expression of the autophagy gene LC3A and of SIRT1 before breakfast (p < 0.04), raised MTOR in the evening (p = 0.007), and lowered mean 24-hour glucose by 4 mg/dL and glycaemic excursions by 12 mg/dL. Gene expression in blood cells over four days, not autophagic flux in muscle or liver.

  5. Study in people2023
    Eight-hour time-restricted eating does not lower daily myofibrillar protein synthesis rates: A randomized control trial

    Obesity (Silver Spring)

    Cited here for how lean mass is lost. 18 men eating about 1.0 g of protein per kg a day spent 10 days on a short eating window or a 12-hour one. Daily muscle building was the same in both groups. Yet the short window group lost more lean mass, 1.0 kg against 0.2 kg.

    Cited here for the lean mass mechanism. 18 men at about 1.0 g/kg/day protein, 10 days of a compressed window versus a 12-hour control. Integrated daily myofibrillar protein synthesis was identical (1.28 versus 1.26 %/day, p = 0.82) while lean mass loss was greater in the compressed arm (-1.0 versus -0.2 kg, p = 0.01).

  6. Study in people2021
    A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults

    Science Translational Medicine

    Cited here as the counterweight to Sutton. 36 lean, healthy adults spent 3 weeks in three groups built to separate fasting from simply eating less. Once calories were matched, fasting did nothing extra for blood markers after meals, gut hormones or fat tissue genes.

    Cited here as the counterweight to Sutton. 36 lean healthy adults, 3 weeks, three arms designed to separate fasting from energy deficit. No fasting-specific effects on postprandial cardiometabolic indices, gut hormones or adipose gene expression once energy was matched (all P > 0.05).

What people report

These are things people say about their own fasting. Nobody checked them, so they are not evidence. They are here because they tell you what to expect and what to watch for, which the studies do not. They cannot tell you whether anything works.

  • Getting enough protein inside six hours is the hardest part.
  • Appetite going down, not up, after the first weeks.
  • Bigger, fewer meals, which suits some people and not others.
  • Evening social meals becoming the main scheduling headache.

Sources: These are self-reports with no comparison group. They come from what people said and did inside the studies on this page. That includes eating about 550 fewer calories a day on their own, and sticking with it just as well across 8 weeks, in Cienfuegos 2020. A separate review of what fasting communities report is still being put together, and nothing here comes from it.

Who this is wrong for

  • Anyone taking an SGLT2 inhibitor, unless a prescriber stops it first. It can cause ketoacidosis (a dangerous acid build-up) while blood sugar looks normal.
  • Anyone on insulin, a sulfonylurea or a meglitinide, unless a prescriber changes the dose for a six-hour eating window.
  • Pregnancy and breastfeeding.
  • Anyone with a past eating disorder, or who now has episodes of eating they cannot control. A six-hour rule is a tighter rule. What separates a diet habit from a disorder is the rule getting tighter and tighter, not how long the fast is.
  • Children and teens.
  • Anyone with a BMI under 18.5, or who meets the UK NICE risk list for refeeding problems.
  • Older adults at risk of muscle loss with age. Each meal needs more protein to switch on muscle building, and a six-hour window gives fewer meals to do it. No eating window study in older adults has looked mainly at lean mass.
  • Anyone who cannot reliably hit their protein target inside six hours. That is what Parr 2023 measured, and people lost more lean mass.

Questions

Is a 6-hour window better than an 8-hour one?
No study has put them head to head. What has been compared is 6 hours against 4 hours, in Cienfuegos 2020. Both lost the same weight, 3.2%. The difference was what kind of tissue came off. The 6-hour group lost 1.5 kg of lean mass and 1.4 kg of fat. The 4-hour group lost 0.8 kg of lean mass and 2.8 kg of fat.
Does 18 hours reach autophagy?
One study had 11 overweight adults try an 8am to 2pm window for four days. It found more activity of the clean-up gene LC3A in their blood cells. That is a real measurement in people at this window length. But it is gene activity in blood cells over four days, not the clean-up process itself in muscle or liver, and the window was early in the day. No study shows that 18 hours, or any set hour, is when autophagy switches on.
Can I hit my protein target in six hours?
That is the real question this window raises. The evidence in people says big servings are not wasted. After a full-body weight workout, 100 g of protein gave a bigger and longer muscle-building response than 25 g, lasting more than 12 hours. It also says the short-window studies that kept lean mass fed people 1.6 to 2.2 g of protein per kg of body weight a day, and the one at 1.0 g per kg a day did not.