The term 'sleep-inducing substance' can be defined by four crossword solutions: SEDATIVE (8 letters), NARCOTIC (8 letters), OPIATE (6 letters), and SOPORIFIC (9 letters). Sedatives are drugs that reduce anxiety and induce relaxation, while narcotics are drugs that produce insensitivity to pain. Opiates are a type of NARCOTIC derived from the opium poppy plant, and soporifics are substances that induce sleep. All of these terms refer to substances that can be used to induce sleep.