Circadian rhythm
The body's internal clock of about 24 hours that regulates sleep, body temperature and metabolism, and that mealtimes influence as much as light does.
The circadian rhythm is an internal clock of about 24 hours, driven by a structure in the hypothalamus and synchronised first and foremost by light. It governs the sleep-wake cycle, body temperature, hormone secretion and, of interest to nutrition, insulin sensitivity and digestion.
Glucose tolerance is not the same at every hour. It is better in the morning and declines through the day: the same meal does not produce the same blood sugar response at 8 am and at 10 pm. This is one of the physiological arguments for a dinner that is neither too late nor too heavy in fast-release sugars.
The second point is less well known: mealtimes are themselves a synchroniser. Light sets the central clock, but the peripheral clocks of the liver, pancreas and gut are largely set by food intake. Eating at very irregular times throws these clocks out of step with each other, a phenomenon seen notably in night-shift workers.
Certain nutrients interact directly with this system. Caffeine delays falling asleep by blocking adenosine, the molecule of sleep pressure; tryptophan is the precursor of serotonin and then melatonin. This is one of the foundations of the link between diet and sleep quality, though no food acts as a sleeping pill.
The central clock has a name: the suprachiasmatic nucleus, in the hypothalamus. But every organ has its own (liver, gut, muscles), and the gut microbiome also follows a circadian rhythm, influenced by mealtimes. It is a three-way dialogue between clock, diet and bacteria.
The best-identified disruptors are artificial light in the evening, which blocks melatonin and delays the clock, late and irregular meals, and night work, which is associated with increased metabolic risk. Conversely, three habits help: getting natural light in the morning, eating an early, light dinner, and keeping regular hours. This is also why an eating window placed earlier in the day seems to give better metabolic results with intermittent fasting.