Twice a year, daylight saving time reminds us that our bodies keep time better than our clocks. Losing or gaining an hour leaves many of us groggy, hungrier at odd times, less focused, and generally "off" for several days. Jet lag does the same. That temporary mismatch occurs because our internal circadian clock doesn't reset instantly when the clock on the wall does.
The same kind of mismatch can happen at the dinner table. While we tend to think that calories are all that matter, our bodies also care when those calories arrive. A late-night meal can put our digestive and metabolic organs on "day shift" while the brain is already signaling that it is nighttime. Like perpetual mini-daylight saving time, the result is biological confusion that subtly changes how we handle glucose, burn fat, regulate hunger, and ultimately use and store energy. Meal timing is another way we synchronize or desynchronize our internal clocks.
The Metabolic Jet Lag of Late Eating
In a study of 20 healthy young adults comparing a dinner at 6 PM for one at 10 PM, the participants had a fixed sleep interval from 11 PM to 7 AM and identical isocaloric dinners. Glucose and fat metabolism were altered along with cortisol levels.
Glucose peak levels were higher for a longer period and even led to higher levels the next day at breakfast.
- Less fat was mobilized and used by the mitochondria for energy, resulting in a net gain and storage of fat.
- Cortisol naturally ebbs to its lowest daily trough in the early hours of sleep, facilitating restorative tissue repair and parasympathetic dominance. Late eating raises this nocturnal baseline, contributing to the glucose alterations.
- These impairments were more pronounced in individuals who would be characterized as “early to bed,” whose baseline internal clocks wake them early, making those late-night meals occur deep within their normal sleep cycle.
A similar, although more rigorous, randomized trial involved 16 overweight adults, with each participant crossing over to eat both an early meal and a 4-hour later meal. In addition to the increased fat storage seen in the first study, they found that late eating doubled the odds of reporting “waketime” hunger, altering the ratio of ghrelin, the “hunger” hormonal signal, and leptin, the “satiety” signal, creating a potent hunger signal. It also reduced the next day’s energy expenditure – all changes pushing us to eat and store more.
In both studies, there were no changes in sleep duration, efficiency, or stage distribution, suggesting that the changes in our energy expenditure were driven by metabolic, endocrine, and gene expression, not by sleep disruption.
Finally, a review provides the broader circadian picture. Our bodies respond to their own circadian clocks. Located deep within our brains, the suprachiasmatic nucleus (SCN) of the hypothalamus controls our central clock, dictating our sleep-wake cycles, our core temperature, which accounts for more than half our energy output, and the secretion of melatonin. The SCN is primarily controlled by “environmental light,” a fancy way of saying the rising and setting of the sun – although electrification and Edison’s light bulb have already perturbed that cycle.
We also have autonomous clocks within our metabolic tissues, including the stomach, intestines, liver, pancreas, and adipose (fat) tissue. Food intake is the external synchronizer for these peripheral clocks. The result of late-night eating is “a failure to communicate,” or at least synchronize.
Eating late at night signals our metabolic organs to enter active digestion, while the brain is signaling biological night. The chemical signal of that night is melatonin, whose levels begin to rise about an hour before our normal bedtime, known as the dim light melatonin onset (DLMO). Melatonin helps prepare for overnight fasting by reducing glucose-stimulated insulin. In the presence of food, this results in elevated glucose levels. Melatonin also enhances our nighttime fat utilization to maintain body temperature and normal function. High melatonin levels in the presence of food shift metabolism toward fat storage.
The lack of synchronization with late meals is governed primarily by what our bodies consider late. For early-to-rise, early chronotypes that release melatonin, the DLMO comes early at around 7 PM. For the late chronotypes, it may not occur till 1 AM. Consuming a large proportion of daily calories near our biological night, the DLMO, causes desynchronization, which is strongly associated with higher body fat percentage and obesity, independent of total caloric intake or physical activity. [1]
Why Early Birds Win
Long before it became trendy, the oldsters were eating early. The early-bird specials were populated by older adults, not just because they offered a reduced price. Nighttime peak and overall melatonin production fluctuate before stabilizing in young adulthood. But with age, those levels decline, blunting the signal and contributing to fragmented sleep patterns and early-morning awakening observed in older adults.
The DLMO, which had shifted later in teens and young adults to 11 PM or later, begins to shift earlier as we age – the over 60s frequently have a DLMO of 6 to 8 PM, with the biological night increasingly earlier.
Meanwhile, the peripheral metabolic clocks are also aging, dampening their signals and muting downstream patterns of insulin sensitivity, digestion, and mitochondrial respiratory capacity. As the strength of our diurnal circadian rhythm declines, it alters the transcription of our genes, and our metabolic clocks drift further from our biological night and day. They lose their flexibility to respond and adapt, becoming more responsive to stress-management signals related to inflammation and DNA repair. Food increasingly becomes their primary synchronizer.
From a metabolic point of view, eating early is healthier for aging adults and for the early risers in general. For the late-to-bed teens and young adults, later meals do not cause the desynchronization of signals it does for others – enjoy that 2 AM slice, ice cream, or late-night cookie run.
Putting the Science on Your Plate
If you are looking for science-based advice, consider the following:
- Front-load your calories, consuming the bulk during your biological day.
- Respect the DLMO; close your eating window 2 to 3 hours before your normal bedtime.
- Tailor your meal timing to your biological clock; for most adults, especially the elderly, earlier is better.
- Use a set meal schedule to keep your peripheral and central clocks in sync.
[1] The interval between an individual's caloric midpoint (the average time at which 50% of daily calories have been consumed) and their biological night (DLMO) dictates metabolic risk. That may be the science behind the aphorism "Eat breakfast like a king, lunch like a prince, and dinner like a pauper."
Chuck Dinerstein, MD, MBA
Director of Medicine
Dr. Charles Dinerstein, M.D., MBA, FACS is Director of Medicine at the American Council on Science and Health. He has over 25 years of experience as a vascular surgeon.
