Which of the following is a sign that someone is abusing stimulants?

Heightened alertness and mood swings are indeed consistent signs of stimulant abuse. Stimulants, such as amphetamines and cocaine, are known for their ability to increase energy levels and improve focus temporarily. This heightened state of alertness can lead to significant mood fluctuations as the body reacts to the chemical imbalances caused by these substances. Therefore, an individual abusing stimulants may experience not only increased wakefulness but also periods of irritability, anxiety, or euphoria, which contributes to the mood swings indicated in the correct choice.

In contrast, the other options reflect states that are generally not associated with stimulant use. For instance, increased sleep and relaxation are typically more aligned with depressants rather than stimulants. Decreased appetite and focus may sometimes occur, but they do not fully capture the alertness aspect of stimulant abuse, and improved concentration and clarity could indicate a temporary positive effect of stimulants but do not encompass the overall behavioral changes, including the negative consequences such as mood swings. Thus, the correct understanding encapsulates the dual effects of increased stimulation and emotional volatility indicative of stimulant abuse.