Caffeine Disrupts Deep Sleep Even When You Fall Asleep Normally, New Study Finds
A new study found that caffeine reduces the slow brain waves needed for deep, restorative sleep even in people who fall asleep quickly and stay in bed for eight hours. Researchers used EEG recordings to measure brain activity and found that caffeine intake significantly reduced the quality of sleep, not just the ability to fall asleep.
Caffeine disrupts the quality of sleep even in people who fall asleep normally and get a full eight hours in bed, according to a new study reported by Science Daily on September 22, 2026.
Researchers used EEG recordings to measure brain activity during sleep in participants who had consumed caffeine. They found that caffeine significantly reduced slow brain waves, which are the deep oscillations that occur during the most restorative stages of sleep. These slow waves are associated with memory consolidation, physical recovery, and immune function.
The findings challenge a common assumption that caffeine only affects sleep if it prevents a person from falling asleep. Many people believe that if they can fall asleep after drinking coffee or tea, the caffeine has not affected their rest. The new research suggests that is not the case.
Participants in the study who consumed caffeine showed reduced slow-wave activity even when their total sleep time and sleep onset were similar to those who had not consumed caffeine. The researchers said this means people may be getting less restorative sleep than they realize.
The study did not specify a cutoff time for caffeine consumption, but researchers noted that caffeine has a half-life of roughly five to six hours in most adults, meaning a cup of coffee consumed in the afternoon can still be active in the body at bedtime.
Health experts have long recommended limiting caffeine in the hours before sleep, but the new findings suggest the window may need to be longer than previously thought. Researchers said more work is needed to determine exactly how many hours before bed caffeine should be avoided to protect sleep quality.
