According to Buddy T,

"Sleeping pills are depressant medications. They act upon the central nervous system to slow down the body’s function. They are classified as "sedative hypnotics," and are prescribed to ease anxiety or enable sleep. The two main types of sedatives are barbiturates and benzodiazepines."

This article explains the dangers of overdosing on sedatives or a combination of sedatives and other drugs such as opioids. 

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