Caffeine has a positive impact on short-term and long-term memory. There's been a lot of past research on caffeine's effects on short-term memory, but recent studies are indicating benefits for long-term memory, as well. "Around 300 to 400 milligrams per day may protect against cognitive decline and Alzheimer's," says Jaclyn London, MS, R.D., a senior dietician at Mount Sinai Hospital in New York City. There is one caveat, though: "Getting enough sleep is also important in Alzheimer's prevention, so keep that in mind if you're caffeine-sensitive.