Stimulants are medications that aid alertness by increasing activity in the central nervous system. They work by increasing levels of certain chemicals in the brain, such as dopamine and norepinephrine, which help to improve focus, attention, and wakefulness. Stimulants are commonly prescribed for conditions such as attention deficit hyperactivity disorder (ADHD) and narcolepsy. They can help individuals stay awake, improve cognitive function, and enhance overall alertness. Therefore, stimulants are the correct answer in this context.