Final answer:
Withdrawal is the term used to describe the symptoms experienced by people who stop using a substance after using it habitually. These symptoms can include fatigue and trouble sleeping. Physical dependence is a related concept, where the user experiences withdrawal upon cessation of drug use.
Step-by-step explanation:
When people who had been using a substance habitually stop using it, they can experience symptoms such as fatigue and trouble sleeping. This is called withdrawal. Drug withdrawal refers to the negative symptoms that occur when drug use is discontinued, and it often includes symptoms that are opposite to the effects of the drug. Physical dependence is another term used to describe this phenomenon, where the user will experience withdrawal from the drug upon cessation of use.