Carlos Solis
Sometimes eight hours of sleep still isn’t enough to feel refreshed in the morning, and we reach for … Researchers say the problem isn’t only how long we sleep but the quality of that sleep — especially slow-wave activity, the brain rhythms that mark deep, restorative sleep. And often undermines that quality, producing poor sleep and morning tiredness.
How Caffeine Alters Sleep
Donata Kurpas, who wrote a review at Wroclaw Medical University, explains in a paper in the journal Nutrients that caffeine can do more than shorten total sleep time or make it harder to fall asleep. It can also reduce slow-wave brain activity and shift the electroencephalogram (EEG) toward a pattern that looks closer to wakefulness.
“Caffeine can shorten sleep time or make falling asleep harder. But even when total sleep time looks normal, slow-wave activity can decline and the EEG can shift toward a more alert brain state,” says Donata Kurpas.
The researchers analyzed data from 32 studies published between 1980 and 2026. In most studies, consuming caffeine suppressed the low-frequency activity that characterizes deep sleep. That produced an EEG sleep profile that was lighter, more aroused, and more like wakefulness — especially during the early hours of NREM sleep and during recovery sleep after sleep deprivation.
A Vicious Cycle
Caffeine’s effects vary widely between people, but the research draws some clear conclusions:
- Sleep quality matters more than duration: you can spend eight hours in bed, but your brain won’t fully recover if slow-wave activity drops.
- Caffeine’s effects are highly individual: genetics, metabolic rate, age, stress, and chronic fatigue all shape how caffeine affects people.
- Caffeine boosts daytime alertness, but sometimes that’s like borrowing energy at the cost of nighttime recovery — creating a loop: more fatigue → more stimulation → worse sleep.
- Consider not only total daily caffeine but also how long before bedtime you consume it, so the body has time to metabolize the stimulant — the review author emphasizes this.
So if you drink coffee regularly and feel chronically tired, don’t assume the problem is only the number of hours you spend in bed. New evidence shows that even with a normal sleep duration, internal sleep quality and the brain’s ability to recover can suffer because of caffeine.
Based on reporting in The Independent
Photo: Unsplash
