Our number one culprit is coffee. Caffeine is a drug which acts as a stimulant and induces alertness. Great if you want to stay awake, but terrible for a good night’s sleep. Coffee has a half life of between three and five hours. The half life is a measure of how long it takes the body to remove half of the substance from your body. Even after five hours you will still have a significant amount of caffeine remaining in your system.