What Time Is Breakfast, Really? Why the Clock May Be Misleading

Imagine two people.

Anna wakes at 5:30 a.m. and eats breakfast at 8:30.

Ben wakes at 8:30 a.m. and eats at 9:00.

Who eats breakfast later?

By the clock, Ben does.

But Anna has already been awake for three hours, while Ben has been awake for only 30 minutes.

If we measure time from the start of each person’s day rather than from midnight, the answer flips.

That may sound like a technicality. It may also be one reason breakfast research has produced such muddled results.

For years, studies have asked whether breakfast is good or bad, whether skipping it leads to weight gain, and whether earlier eating is healthier.

But there is a more basic question:

What does “early” or “late” breakfast actually mean when people wake at very different times?

Even researchers do not fully agree on what breakfast is

There is no single operational definition of breakfast.

In one study, breakfast is the first meal after the longest period of sleep. In another, it must be eaten within two or three hours of waking. In a third, researchers simply draw a line on the clock.

This is not a trivial methodological detail.

In the International Breakfast Research Initiative, national dietary datasets had to be handled differently because the available definitions were not compatible. In the UK dataset, for example, breakfast was defined by a morning time window. In the US dataset, participants identified an eating occasion as breakfast themselves, with an energy threshold added by the researchers. Other countries required still other definitions. The authors explicitly noted that a single universal breakfast definition could not be applied across all datasets.^[1]

The problem

If one study defines “breakfast skipping” as eating nothing before 9 a.m., while another defines it as eating nothing for several hours after waking, the two studies may be examining different physiological situations.

There is an additional caveat worth mentioning. The International Breakfast Research Initiative received industry support, including from breakfast-food manufacturers. That makes it a poor source for judging whether breakfast is beneficial. Here, however, we are using it only for a narrower and well-documented point: researchers themselves had difficulty defining breakfast consistently.

We replaced the wall clock with time since waking

We analysed data from the 2017–2018 US National Health and Nutrition Examination Survey (NHANES), which combines detailed dietary recalls with sleep information.

Instead of looking only at the clock time of the first caloric intake, we also calculated how long each person had been awake before eating.

The difference was striking.

People with later sleep timing ate their first calories substantially later by the clock. But they did not wait substantially longer after waking.

Across the two dietary-recall days, the correlation between chronotype and the clock time of the first caloric intake was about 0.40–0.41. The corresponding correlation with the delay from waking to first food was essentially zero: about −0.03 to −0.04.

The pattern remained when we changed the calorie threshold used to define the first meaningful eating occasion.

Our main finding

Later chronotypes eat later by the clock.

They do not, on average, eat much later relative to their own waking time.

Some of what we call “late breakfast” may therefore be little more than late waking expressed on a wall clock.

This is not yet a finding about health. It does not tell us which schedule is better.

It tells us that the two ways of measuring meal timing are not interchangeable.

Weekends gave us an even cleaner test

One way to reduce differences between people is to compare each person with themselves.

About 1,664 NHANES participants happened to have one dietary recall on a weekday and another on a weekend.

On weekends, people generally woke later. Their first meal also moved later.

The median shifts were approximately:

• wake time: +30 minutes;
• first meal: +30 minutes;
• delay from waking to first food: 0 minutes.

In other words, people tended to move the whole schedule together.

The clock moved. The behaviour relative to waking barely did.

For us, this is currently the most compelling result in this branch of the analysis.

Not because it proves that early breakfast is healthy. It does not.

It suggests something more basic: ordinary clock time may sometimes describe a social schedule better than it describes the physiological position of a meal within a person’s day.

But waking time is not the body’s real clock either

The body has its own circadian timing system.

One of the best laboratory markers of circadian phase is the point in the evening when melatonin begins to rise under dim-light conditions. Researchers call this dim-light melatonin onset, or DLMO. In plain language, it is a useful marker of the beginning of the body’s biological night.

That moment does not occur at the same clock time for everyone.

In a study of 110 young adults, people with higher body fat tended to consume the midpoint of their daily calories about 1.1 hours closer to the start of their biological night than leaner participants. Ordinary clock time did not distinguish the groups in the same way.^[2]

This was an observational study, so it cannot show that eating later in circadian time caused higher body fat.

But it demonstrates the measurement problem clearly:

The same time on the clock does not necessarily mean the same biological time.

The body really can respond differently at different biological times

At this point, an obvious objection appears: fine, there are different clocks. But does that difference matter physiologically?

Apparently, it can.

In a large randomised crossover experiment involving 845 participants, each person completed the same test twice: after an eight-hour fast, they consumed a drink containing 75 grams of glucose.

One test took place four hours before the participant’s usual bedtime. The other took place only one hour before bedtime.

In the later condition, melatonin concentrations were about 3.5 times higher. Insulin exposure was about 6.7% lower, while glucose exposure was about 8.3% higher.^[3]

Same glucose load.

Same fasting duration.

Same person.

Different position within the biological evening.

This is a causal experiment.

It does not prove that a late breakfast causes diabetes or obesity.

It does show that the metabolic response to the same food can depend on where that meal falls within the biological day.

That also means “16:8” can describe very different experiments

The same problem appears in time-restricted eating.

Imagine two people who both fast for 16 hours.

One finishes dinner early and eats breakfast in the morning.

The other eats late into the evening and then skips breakfast.

On paper, both are practising 16:8.

Physiologically, the 16 fasting hours occupy very different parts of the day.

A small but tightly controlled experiment compared breakfast skipping with dinner skipping. Seventeen people completed different experimental days while total daily energy intake was controlled.

Average 24-hour glucose did not differ substantially. But after breakfast skipping, the glucose response to lunch was about 46% higher than after dinner skipping, and one measure of post-meal insulin resistance differed by about 54%.^[4]

This was a one-day experiment in only 17 people. It cannot support a rule such as “never skip breakfast”.

But it undermines another rule:

Sixteen hours without food in the morning and sixteen hours without food in the evening are not necessarily the same physiological intervention.

What do time-restricted eating studies show?

Here the evidence becomes less tidy.

In one well-known experiment, men with prediabetes ate within a six-hour window and finished all food before 3 p.m. Energy intake was controlled so that weight loss would not explain the results.

After five weeks, insulin sensitivity, pancreatic beta-cell responsiveness, blood pressure and several other markers improved.^[5]

That sounds like a victory for early eating.

But there is a problem: the intervention changed two things at once.

• The participants fasted for longer.
• They also moved their food intake earlier in the day.

The experiment cannot cleanly tell us which component mattered most.

Another crossover study helps to untangle the two factors. Fifteen men tried two nine-hour eating windows:

• 8 a.m. to 5 p.m.;
• 12 p.m. to 9 p.m.

Both schedules improved glucose tolerance. The study did not find a clear overall metabolic advantage for the earlier window.^[6]

So the slogan “the earlier, the better” goes beyond the evidence.

Fasting duration and the circadian placement of food are different variables, yet many interventions change both simultaneously.

What did we find when we looked at health outcomes?

We also asked whether the delay from waking to the first substantial meal was associated with measurable health outcomes in NHANES.

In our preliminary analysis, after adjustment for age, sex, race and ethnicity, income, energy intake, sleep duration, chronotype and social jetlag, each additional hour between waking and the first eating occasion of at least 100 kcal was associated with roughly:

• +0.25 units of body mass index (BMI);
• +0.6 cm of waist circumference.

The waist association remained similar when we changed the calorie threshold used to define the first substantial meal.

Other outcomes were much less impressive.

HbA1c, a marker of average blood glucose over the previous few months, showed essentially no relationship with first-meal timing. An apparent association with a blood marker of inflammation disappeared when we changed the meal definition. Daytime sleepiness was also effectively unrelated to the delay.

And there is a major problem with interpreting the BMI and waist results.

A person with excess weight may deliberately delay or skip breakfast because they are trying to lose weight. In that case, the apparent direction of cause is reversed.

Our current model still needs stronger adjustment for weight-loss attempts, physical activity, smoking, diet quality and shift work.

What our data do NOT show

We did not find that waiting longer after waking causes a larger waist.

We found an association.

The causal direction may run the other way, or both may be influenced by other factors.

There is another inconvenient detail: wake-relative timing currently predicts BMI and waist only slightly better than ordinary clock time.

So we cannot yet claim that we have found the “correct” clock.

That may be the most interesting result

We started with one question:

Should you eat breakfast?

Then came a better one:

When should you eat breakfast?

But the data pushed us towards a third:

Relative to which clock should “when” be measured?

At least three different clocks are now in play.

1. Wall-clock time

Breakfast at 8 a.m., 10 a.m. or noon.

It is convenient. But 10 a.m. can mean four hours after waking for one person and twenty minutes after waking for another.

2. Sleep-relative time

Thirty minutes after waking. Three hours after waking. Two hours before sleep.

This scale is personal and easy to measure.

3. Circadian time

The position of a meal relative to the body’s internal clock, which researchers can estimate using markers such as the evening rise in melatonin.

This may be physiologically the most interesting scale, but it is much harder to measure outside a laboratory.

In short

“Breakfast at 10 a.m.” is not a complete physiological description.

At the very least, we also need to know when the person woke up.

In an ideal study, we would also know where that meal falls relative to the person’s internal circadian clock.

So, should you eat breakfast?

There is still no convincing universal answer.

Randomised breakfast trials do not support the simple claim that everyone should eat breakfast.

A meta-analysis of seven trials involving 425 participants found that people assigned to skip breakfast lost about half a kilogram more over the study periods, while their LDL cholesterol rose modestly. Most other cardiometabolic outcomes did not differ clearly.^[7]

Another systematic review of randomised trials likewise found no good evidence that adding breakfast is an effective weight-loss strategy, while emphasising that the trials were generally short and the quality of evidence limited.^[8]

So the advice:

“Everyone should eat breakfast.”

does not follow from strong experimental evidence.

But neither does the opposite advice:

“Everyone is better off skipping breakfast.”

Perhaps the binary question itself is too crude.

What can you use from this now?

There is no evidence-based formula that can calculate your ideal breakfast time to the minute.

But there is a more precise way to think about your own schedule.

Instead of asking:

“What time do I eat breakfast?”

ask:

“How long after waking do I first eat a substantial meal?”

and:

“Where does most of my food intake fall relative to my sleep?”

These are not prescriptions for better health.

They are simply more physiologically informative descriptions of meal timing.

And they may be a better place for research to start.

The central idea

For decades, nutrition research has tried to determine whether breakfast is good for us.

Before recruiting another enormous cohort, the next good breakfast study may need something more basic:

a better clock.

References

1. Gibney MJ, Barr SI, Bellisle F, et al. Breakfast in Human Nutrition: The International Breakfast Research Initiative. Nutrients. 2018;10(5):559. DOI: 10.3390/nu10050559.
2. McHill AW, Phillips AJK, Czeisler CA, et al. Later circadian timing of food intake is associated with increased body fat. American Journal of Clinical Nutrition. 2017;106:1213–1219. DOI: 10.3945/ajcn.117.161588.
3. Garaulet M, Lopez-Minguez J, Dashti HS, et al. Interplay of Dinner Timing and MTNR1B Type 2 Diabetes Risk Variant on Glucose Tolerance and Insulin Secretion: A Randomized Crossover Trial. Diabetes Care. 2022;45:512–519. DOI: 10.2337/dc21-1314.
4. Nas A, Mirza N, Hägele F, et al. Impact of breakfast skipping compared with dinner skipping on regulation of energy balance and metabolic risk. American Journal of Clinical Nutrition. 2017;105:1351–1361. DOI: 10.3945/ajcn.116.151332.
5. Sutton EF, Beyl R, Early KS, et al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism. 2018;27:1212–1221.e3. DOI: 10.1016/j.cmet.2018.04.010.
6. Hutchison AT, Regmi P, Manoogian ENC, et al. Time-Restricted Feeding Improves Glucose Tolerance in Men at Risk for Type 2 Diabetes: A Randomized Crossover Trial. Obesity. 2019;27:724–732. DOI: 10.1002/oby.22449.
7. Bonnet JP, Cardel MI, Cellini J, Hu FB, Guasch-Ferré M. Breakfast Skipping, Body Composition, and Cardiometabolic Risk: A Systematic Review and Meta-Analysis of Randomized Trials. Obesity. 2020;28:1098–1109. DOI: 10.1002/oby.22791.
8. Sievert K, Hussain SM, Page MJ, et al. Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials. BMJ. 2019;364:l42. DOI: 10.1136/bmj.l42.

About our NHANES analysis

The NHANES 2017–2018 results reported above are our own exploratory analyses, not results from a peer-reviewed publication. They should be treated as preliminary until the full sensitivity analysis is completed and the calculations are independently reproduced.