When I eat a candy bar, even a small one, I get sleepy. Chocolate is the primary consistancy between the candy bars. If I eat even a bite of a candy bar with chocolate I get sleepy within a few minutes. Is there any reason for that? Is it the caffeine?
I'm sure you've found an answer to your question already, but I thought I'd answer this anyway, in case it helps someone else. There are many reasons that chocolate could make you tired. One of the most obvious is that chocolate has a lot of sugar, which can cause a blood sugar spike and nasty drop for people who have insulin resistance or diabetes. That usually takes a little while to notice, however, and you say your symptoms start immediately, which is curious. Dark chocolate contains serotonin, which aids sleep and relaxation, but milk chocolate contains stimulants. In short, I'm not sure why you're having this reaction, but if it worries you you should talk to your doctor about it. In the meantime, you should consider steering clear of chocolate (as hard as that can be!)
Good luck! Come back and update us on what you've found if you have a chance.
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Has been stated if you have add or adhd. You're more susceptible to falling asleep. Chocolate, coffee, caffeine based products due to these being stimulants, affects serotonin and other brain chemicals. You may also feel sluggish for a few days as well.