Some people can have caffeine late in the day and fall asleep easily, while others may remain awake for hours.
But researchers are increasingly finding that simply asking whether coffee delays sleep may miss a more important issue: what caffeine does to the brain after sleep begins.
Researchers from Wroclaw Medical University have found that caffeine does not always affect sleep in obvious ways. A person may still fall asleep at a normal time and remain asleep for what appears to be a full night, yet the quality of that rest may still be altered.
“Caffeine may shorten sleep or make it more difficult to fall asleep; however, even when sleep duration appears normal, it may reduce slow-wave activity and shift the EEG pattern toward a more ‘wakeful’ brain,” says lead researcher Prof. Kurpas.
How Caffeine Affects The Quality Of Sleep
For the study, the researchers used EEG, or electroencephalography, a method used to record the brain’s electrical activity. EEG measurements can reveal much more than how long someone sleeps or how often they wake up. They can also provide clues about the biological quality and depth of that sleep.
“EEG allows us to see not only whether a person is sleeping, but also how the brain is sleeping. Classical sleep assessment assesses sleep duration and its stages, whereas quantitative EEG analysis reveals more subtle changes, such as reduced slow-wave activity, which is an important marker of sleep depth and its restorative character,” explains Prof. Donata Kurpa.
The Importance of Slow Waves
Slow waves are a major feature of deep sleep, the stage associated with physical recovery, replenishing energy reserves, and maintaining healthy brain function. “Caffeine may shorten sleep or make it more difficult to fall asleep; however, even when sleep duration appears normal, it may reduce slow-wave activity and shift the EEG pattern toward a more ‘wakeful’ brain,” says Prof. Kurpas.
In practical terms, someone might spend eight hours in bed without getting the same level of neurological recovery they would normally expect. Because the disruption may not cause obvious awakenings, it can easily go unnoticed. “The subjective feeling of having slept well does not always correspond to what we observe in neurophysiological recordings. A person may fall asleep without major difficulty and not remember awakenings, while the brain may display fewer features of deep sleep,” says Prof. Kurpas.
How Coffee Can Impact Sleep
Another major finding from caffeine research is that responses vary considerably from person to person. Genetics, metabolism, age, stress, and chronic fatigue can all influence how strongly caffeine affects sleep. For some people, even coffee consumed early in the day may still have an impact by nighttime. “It is not only about coffee consumed just before bedtime. For some people, the total amount of caffeine consumed during the day and whether the body has enough time to metabolise it before nightfall may also be important,” Prof. Kurpas emphasises.
“If caffeine helps a person function during the day while simultaneously worsening the quality of nighttime recovery, a vicious circle may develop: greater fatigue, greater need for stimulation, and poorer sleep,” says Prof. Kurpas. “Caffeine is neither ‘good’ nor ‘bad’. It is a biologically active substance whose effects depend on dose, time of day, age, lifestyle, sleep quality, stress burden, and individual sensitivity.”
