Caffeine is a stimulant and affects sleep adversely, according to studies. It stays in your body for more than seven hours. Even if you are able to fall asleep, you may not enter the deep sleep phase, which is when your brain really rests. Alcohol is eliminated from the body rapidly and causes withdrawal symptoms two or three hours later. This has a negative reaction. People fall asleep more quickly under the influence of alcohol, but alcohol reduces rapid eye movement (REM), which is when you dream and actually rest.