For years, people have debated whether drinking coffee at night interferes with sleep. Some people can consume caffeine at the end of the day and fall asleep easily, while others can stay awake for hours. But researchers are increasingly finding that simply asking whether coffee delays sleep may miss a more important question: what caffeine does to the brain once sleep begins.
To investigate those effects, scientists are using EEG, or electroencephalography, a method used to record electrical activity in the brain. EEG measurements can reveal much more than how long a person 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 sleeps. Classical sleep assessment evaluates the duration of sleep and its stages, while quantitative analysis of the EEG reveals more subtle changes, such as the reduction of slow wave activity, which is an important marker of the depth of sleep and its restorative nature,” explains Professor Donata Kurpas from the Department of Nursing at the Medical University of Wroclaw.
Slow waves are an important feature of deep sleep, the stage associated with physical recovery, replenishing energy stores, and maintaining healthy brain function.
Caffeine May Make Sleep Less Restful
Caffeine doesn’t always affect sleep in obvious ways. A person can still fall asleep at a normal time and stay asleep for what seems like a full night, but the quality of that rest can still be altered.
“Caffeine can shorten sleep or make it more difficult to fall asleep; however, even when sleep duration appears normal, it can reduce slow wave activity and change the EEG pattern towards a more ‘awake’ brain,” says Professor Kurpas.
In practical terms, someone could spend eight hours in bed without getting the same level of neurological recovery they would normally expect. Because the interruption 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 records. A person can fall asleep without major difficulties and not remember their awakenings, while the brain can show fewer characteristics of deep sleep,” adds the expert.
Why caffeine affects people differently
Another important finding from caffeine research is that responses vary considerably from person to person. Genetics, metabolism, age, stress, and chronic fatigue can influence how strongly caffeine affects sleep.
For some people, even coffee consumed early in the day can have an impact overnight.
“It’s not just about the coffee consumed just before bed. For some people, the total amount of caffeine consumed during the day and whether the body has enough time to metabolize it before nightfall may also be important,” emphasizes Professor Kurpas.
That possibility may be especially relevant for people who rely on caffeine to improve focus, endurance or performance, including athletes, people who do mentally demanding jobs, and anyone who regularly consumes coffee or other caffeinated products to stay alert.
Caffeine can create a fatigue cycle
Caffeine can temporarily increase alertness and make fatigue feel less noticeable. However, experts warn that the boost can sometimes come at the expense of the body’s ability to fully recover during sleep.
In that sense, caffeine can be like “borrowing energy” from the body. If nighttime recovery becomes less effective, the person may feel more tired the next day and turn to even more caffeine for help.
“If caffeine helps a person function during the day and at the same time worsens the quality of nighttime recovery, a vicious cycle can develop: increased fatigue, increased need for stimulation and worse sleep,” says Professor Kurpas.
These findings are one reason sleep researchers pay less attention to sleep duration alone. Increasingly, the focus is on what the brain actually does at night and whether that sleep is biologically restorative.
“Caffeine is neither ‘good’ nor ‘bad’. It is a biologically active substance whose effects depend on the dose, time of day, age, lifestyle, quality of sleep, stress load and individual sensitivity,” concludes the expert.