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Caffeine, a stimulant, is a “bad idea for a lot of people because it affects sleep adversely,” Dr. Doghramji says. It stays in your body for more than seven hours, though some people can process it faster than others. “My recommendation is no caffeine after lunch, just to be on the safe side,” he adds. Even if you are able to fall asleep, you may not enter the deep sleep phase, which is when your brain really rests.