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Has drinking green tea before bedtime kept you awake at night? Zen priests used to drink tea to fight with the sleepiness during the meditation. Tea has an effect on sleep as a stimulant. Caffeine stimulates brain and central nervous system, activating brain to improve reactions, and improving mental condition, which means that caffeine is used as a central nervous system stimulant. The amount of caffeine in tea leaves varies with the timing of the harvest. Leaves harvested in May, June, and July contain caffeine most, which is about 2~4% of dried leaves. The amount decreases as leaves grow. Leaves that are plucked in January contain only 0.5% caffeine. Therefore, black tea, power green tea, Gyokuro and high-quality tea, which contain much caffeine have effect on sleep. You are advised to drink tea with a lot of caffeine when you would like to keep yourself awake.

(Fumiko Hayakawa)