Which type of substance is used to increase activity in the nervous system, often taken in the morning?

Stimulants are substances that increase activity in the nervous system, which results in enhanced alertness, attention, and energy levels. Common examples include caffeine, nicotine, and certain prescription medications that are used for conditions like ADHD. Many people consume stimulants, particularly caffeine, in the morning to help wake up and improve their focus as they start their day. This use aligns with the primary characteristic of stimulants: their ability to elevate mood and energy by increasing the activity of neurotransmitters in the brain, such as dopamine and norepinephrine.

In contrast, depressants, barbiturates, and opiates work by slowing down brain activity or producing calming effects, which would not be conducive to the desire to increase activity or alertness in the morning. Thus, stimulants are the correct answer as they are specifically associated with enhancing nervous system activity and are widely used for this purpose.