For many people, coffee is naturally part of their evening routine. However, current research shows that caffeine not only disrupts sleep on the surface but also interferes deeply with our body’s biological control mechanisms. Especially in the context of chronobiology, it becomes clear why drinking coffee late at night is more problematic than has long been assumed.

The Drawbacks of Nighttime Caffeine

Chronobiology deals with the body’s internal clocks, particularly the circadian rhythm—a roughly 24-hour cycle that regulates sleep, hormone production, body temperature, and metabolism. Central to this is the hormone melatonin, which rises in the evening and prepares the body for sleep. This is where caffeine intervenes: it blocks the neurotransmitter adenosine, which signals fatigue, and simultaneously delays the release of melatonin. As a result, the “internal clock” shifts backward—you become tired later and often sleep less deeply.

EEG-based sleep studies, including research on healthy adults in sleep laboratories, show that caffeine increases neural activity during sleep. Even when people fall asleep, the brain remains in a state that resembles light wakefulness rather than restorative deep sleep phases. These deep sleep phases, however, are crucial for physical recovery, immune function, and memory consolidation. In chronobiological terms, this means: The body is technically in sleep mode, while parts of the brain remain active—a kind of “internal desynchronization.” This explains why one does not feel fully rested despite getting enough sleep.

Fruit Fly Study: Impulsive Behavior Observed

A study published in the journal iScience, led by Erick Saldes, Paul Sabandal, and Kyung-An Han, investigated how nighttime caffeine consumption affects impulse control. The fruit fly Drosophila melanogaster served as the model organism, as its genetic and neural systems share important similarities with humans. To investigate the effects more closely, the researchers conducted several experiments:

• Flies were given caffeine in their food at various concentrations
• Intake occurred during the day or at night
• In some cases, the flies were additionally subjected to sleep deprivation

Impulsivity was measured using a simple behavioral paradigm: Flies typically respond to a strong airflow by halting their movement. Saldes explained: “Under normal circumstances, flies stop moving when exposed to a strong airflow. We found that flies that consumed caffeine at night were less able to suppress their movement and exhibited impulsive behavior, such as reckless flying, despite these aversive conditions.” In contrast, the researchers reported that caffeine consumed during the day did not lead to the same type of reckless flight behavior.

Particularly interesting: female flies were more sensitive. Since they lack human hormones such as estrogen, the results suggest that other genetic or physiological factors influence this increased sensitivity. Han emphasized that uncovering these mechanisms could help better understand how nocturnal physiology and sex-specific factors modulate the effects of caffeine. However, the effects observed in the study go even further. Even when people fall asleep after consuming coffee, the brain remains measurably more active. The neural activity patterns resemble a state of light wakefulness rather than restorative deep sleep phases. Yet it is precisely these deep sleep phases that are essential for physical recovery, immune function, and memory consolidation. From a chronobiological and behavioral science perspective, therefore, the following applies: Anyone who wants to maintain their sleep quality, recovery, and cognitive performance in the long term should avoid caffeine several hours before bedtime, especially at night or when sleep is already limited.

https://scitechdaily.com/new-study-reveals-surprising-effect-of-drinking-coffee-at-night/

Caffeine in the Morning Could Help Protect Your Brain in the Long Term

Consuming caffeine at other times of the day, on the other hand, could have positive effects. Research suggests that a morning cup of coffee or tea could prove helpful by quietly promoting your brain health. A long-term study found that moderate consumption of caffeinated coffee or tea was associated with an 18% lower risk of dementia and better long-term cognitive performance. The positive effects were most pronounced with 2–3 cups of coffee or 1–2 cups of tea daily—and even applied to people who are genetically predisposed to dementia.

Early prevention of dementia is particularly important, as current treatment options are limited and generally offer little benefit once symptoms have appeared. Therefore, scientists are increasingly focusing on lifestyle factors, including diet, that could influence the development of cognitive decline. Coffee and tea contain compounds such as polyphenols and caffeine, which are believed to support brain health. These substances may help reduce inflammation and limit cell damage, both of which are associated with cognitive decline. However, previous research on coffee and dementia has yielded mixed results, often due to shorter study periods or limited data on long-term consumption habits and different types of beverages.

Long-Term Data Offers Clearer Insights

The large-scale prospective cohort study, conducted by researchers at Mass General Brigham, the Harvard T.H. Chan School of Public Health, and the Broad Institute of MIT and Harvard, examined data from 131,821 participants in the Nurses’ Health Study (NHS) and the Health Professionals Follow-Up Study (HPFS). Participants were followed for up to 43 years, with repeated assessments of diet, dementia diagnoses, subjective cognitive concerns, and objective cognitive performance. The researchers analyzed how the consumption of caffeinated coffee, tea, and decaffeinated coffee is associated with long-term brain health outcomes. Among the more than 130,000 participants, 11,033 developed dementia over the course of the study. People who consumed larger amounts of caffeinated coffee had an 18% lower risk of developing dementia than those who rarely or never drank it. They also reported lower rates of subjective cognitive impairment (7.8% versus 9.5%) and performed better on certain objective cognitive tests.

Similar patterns were observed among tea drinkers, while decaffeinated coffee showed no such associations. This suggests that caffeine may be an important factor in the observed positive effects on the brain, although further research is needed to confirm the underlying mechanisms. The strongest effects were seen in participants who drank 2–3 cups of caffeinated coffee or 1–2 cups of tea daily. Higher caffeine intake did not appear to have any negative effects. Instead, comparable benefits were observed to those seen with the moderate intake highlighted in the study. “We also compared people with different genetic predispositions to developing dementia and found the same results—meaning that coffee or caffeine is likely equally beneficial for people with high and low genetic risk of developing dementia,” said lead author Yu Zhang, MBBS, MS, a doctoral student at the Harvard Chan School and a research fellow at Mass General Brigham.