Why does coffee make me sleepy immediately?

Online Answer
Summary. If drinking a cup of coffee makes a person feel tired, the effects of caffeine may be responsible. Caffeine increases alertness by interfering with certain chemical processes in the brain that regulate the sleep-wake cycle. However, once the body completely metabolizes caffeine, it can make people feel tired..
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Caffeine occurs naturally in the tea plant, Camellia sinensis, so all brewed tea contains some caffeine. ... Cooler water and shorter steeping time extracts less caffeine—think green or white tea. Tea is the only plant that contains L-theanine, an amino acid that promotes calm and relaxation..
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If caffeine is a big part of your daily diet, taking it away can have a host of unpleasant effects in the short term. These include headache, tiredness, sleepiness, down moods, trouble concentrating, and crankiness. You'll start to feel symptoms a day or two after you stop. They can last anywhere from 2 to 9 days.