Mild stimulants are substances that produce a mild increase in alertness and energy. Caffeine is a mild stimulant that is found in many beverages, such as coffee, tea, and soda. It is also found in some foods, such as chocolate. Caffeine works by blocking the action of adenosine, a neurotransmitter that helps regulate sleep and wakefulness. Caffeine can help improve focus and alertness, but it can also cause side effects such as jitteriness, headaches, and insomnia.