Which of the following symptoms is classified as a physical sign of withdrawal from substances?

Insomnia is classified as a physical sign of withdrawal from substances because it directly relates to physiological changes that occur when a person is reducing or stopping substance intake. Withdrawal from many substances, such as alcohol, opioids, and benzodiazepines, often disrupts normal sleep patterns, which can lead to difficulties falling asleep or maintaining sleep. This insomnia can manifest as both subjective complaints of sleep disturbance and objective observations of restlessness or changes in sleep cycles.

In contrast, increased social involvement, elevated mood, and improved concentration are more behavioral or psychological in nature and do not typically indicate physical withdrawal symptoms. Instead, they might suggest a positive response or adjustment, which is not consistent with the expected symptoms associated with withdrawal. Recognizing the distinction between physical signs, like insomnia, and these other behavioral symptoms is crucial in understanding the broader context of substance withdrawal.