Coffee Can Disrupt Deep Sleep Even Without Keeping You Awake

Coffee does not necessarily have to keep a person awake to interfere with sleep. New research suggests that caffeine may alter the quality of sleep after a person has already fallen asleep, potentially reducing the deep, restorative stage of sleep.

Researchers are using electroencephalography (EEG) to examine how caffeine affects brain activity during sleep. Unlike conventional sleep assessments, which primarily measure sleep duration and stages, quantitative EEG can detect more subtle changes in brain activity, including alterations in slow-wave activity.

"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 Kurpas from the Department of Nursing at Wroclaw Medical University.

Slow waves are a characteristic feature of deep sleep, a stage involved in physical recovery, restoration of energy reserves and healthy brain function.

Caffeine May Reduce Sleep Quality

The effects of caffeine are not always obvious. A person may fall asleep at the usual time, sleep for approximately eight hours and have no clear memory of waking during the night, while caffeine may nevertheless have changed the underlying brain activity.

"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," Kurpas says.

This means that spending a full night in bed does not necessarily guarantee the same level of neurological recovery. Because the changes may occur without noticeable awakenings, people may not realize that their sleep has been affected.

"The subjective feeling of having slept well does not always correspond to what we observe in neurophysiological recordings," Kurpas adds.

Sensitivity to Caffeine Varies

The effect of caffeine can differ substantially between individuals. Factors such as genetics, metabolism, age, stress and chronic fatigue may influence how long caffeine remains active and how strongly it affects sleep.

For some people, caffeine consumed earlier in the day may still influence nighttime sleep.

"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 metabolize it before nightfall may also be important," Kurpas says.

This may be particularly relevant for people who regularly use caffeine to maintain concentration, endurance or alertness, including athletes and people engaged in mentally demanding work.

A Possible Fatigue Cycle

Caffeine can temporarily increase alertness and make fatigue less noticeable, but reduced sleep quality could potentially contribute to tiredness the following day. Some people may then consume more caffeine to compensate, creating a cycle of fatigue, stimulation and disrupted sleep.

"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," Kurpas says.

The findings highlight why sleep quality cannot always be assessed by duration alone. Researchers are increasingly examining brain activity during sleep to determine whether nighttime rest is truly restorative.

"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," Kurpas concludes.