Soporific stuff refers to anything that induces sleepiness or drowsiness. If you are looking for synonyms for this phrase, you could consider using terms like "sleep-inducing substances," "sedative materials," or "slumber-promoting products." Other phrases that could be used include "calming ingredients," "tranquilizing compounds," or "relaxation aids." Depending on the context, you may also be able to use phrases such as "tiring activities," "endurance-testing pursuits," or "mentally exhausting endeavors." Ultimately, there are many words and phrases that can be used to describe soporific stuff, and the specific term you choose will depend on the context and your intent.