What actually happens to the brain in the middle of the day

One of the main reasons lies in the functioning of the systems that regulate wakefulness and sleep. From the moment of waking up, adenosine gradually accumulates in the brain — a substance associated with the increasing need for sleep. This need is called “sleep pressure,” and it constantly grows as long as we are awake. At the same time, circadian rhythms are at work; they set the fluctuations in alertness levels throughout the day. In the morning, this system helps to resist the increasing sleep pressure, but by the beginning of the afternoon, the body enters a natural period of increased susceptibility to sleep. The combination of these two factors creates that very “window of fatigue,” when drowsiness becomes particularly noticeable.

Eating changes the signals between the intestines, metabolism, and the brain: the brain receives information about the stretching of the stomach, the composition of nutrients, and the functioning of intestinal hormones. A larger meal is more often associated with impaired attention and pronounced drowsiness after eating, although it is impossible to definitively identify any single component — fats, carbohydrates, or sugar — as the main culprit.

An interesting role here may be played by the orexin system; these neurons in the hypothalamus help maintain wakefulness and are sensitive to metabolic signals, including glucose levels. It turns out that the act of eating itself can influence the neural circuits responsible for our alertness.

At the same time, recent studies show that drowsiness after eating is not the same as fatigue caused by lack of sleep. In an experiment where scientists measured brain activity and alertness levels before and after meals, afternoon drowsiness was found to be associated with a shift in the activity of the cerebral cortex toward inhibition: reactions became slower, and attention weakened. With sleep deprivation, the picture was different — in the cortex, on the contrary, increased neural excitability was observed. That is, the feeling of fatigue can be very similar, but the brain reaches it through different pathways.

What can be done to cope with this decline? A common method is to drink coffee or tea, but this is not an ideal solution. Caffeine does not give the brain additional energy; it only blocks adenosine receptors, temporarily dampening the signals indicating the need for sleep. At the same time, adenosine itself continues to accumulate, so caffeine only postpones the problem rather than solving it. Moreover, if you drink coffee late in the day, caffeine may interfere with falling asleep in the evening, and the next day the fatigue will be even worse.

A more effective strategy is exposure to light. Daylight is a strong signal for the systems responsible for alertness. A review of 62 experimental studies from 2024 showed that daylight most consistently improves subjective feelings of alertness and working memory. The effect is not limited to sunlight: there is evidence that lamps mimicking the blue spectrum of daylight improve reaction speed, reduce the number of attention lapses, and decrease drowsiness among office workers.

If light and coffee don’t help, you can either fight against the natural sleep pressure or slightly reduce it. A short daytime nap lasting 10–20 minutes is often enough to reduce drowsiness and restore alertness, while remaining in the light sleep phase; this reduces the risk of waking up with “sleep‑induced grogginess”. If you opt for a longer nap, about 60 minutes, it may additionally improve memory, but it increases the likelihood of waking up after a deep sleep and feeling worse for some time.

It is important to remember that an afternoon slump is not a sign of laziness or a drop in motivation. Human cognitive abilities are not designed to maintain consistently high productivity throughout the entire workday: attention, reaction speed, and alertness naturally fluctuate under the influence of biological rhythms. According to the authors, the problem is rather that the modern work schedule is often structured as if these fluctuations did not exist.