Consuming the caffeine equivalent of four cups of coffee up to 12 hours before bedtime reduces deep sleep by 21 minutes—undetectable by the consumer. Digest
Caffeine is the world’s most popular psychoactive drug. Often considered harmless, its influence on sleep can be more disruptive than many realize. A recent study found that while a modest amount of caffeine had little effect on sleep, large amounts of caffeine—roughly the amount in four cups of coffee—reduced deep sleep by 21 minutes when consumed within 12 hours of bedtime.
The study involved 23 healthy young men who typically consumed less than 300 milligrams of caffeine daily. Each participant consumed either a placebo or 100 or 400 milligrams of caffeine, taken twelve, eight, or four hours before bedtime, with 48-hour “washout” periods between each condition. The researchers monitored the participants' sleep using sleep diaries and in-home polysomnography and applied statistical models to examine how dose and timing affected subjective and objective sleep quality.
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The 100-milligram dose didn’t meaningfully disrupt sleep. However, the 400-milligram dose caused measurable changes when taken up to 12 hours before bedtime, including: - Deep sleep decreased by 21 minutes - Light sleep increased by 6% - Time to reach stable sleep increased by 15 minutes Notably, the participants didn’t notice any difference in their sleep quality.
These findings suggest that while smaller doses of caffeine may be sleep-friendly even late in the day, a large dose can interfere with the structure and quality of sleep, even if taken in the morning. Because caffeine consumers might not perceive the disruption, they may be more likely to repeat the cycle, unknowingly trading long-term rest for short-term alertness. Learn about other factors that influence sleep quality in this clip featuring Dr. Matt Walker.