Something that is soporific makes you feel very sleepy or tired.
Soporific things are usually thought of as boring or making people want to sleep.
Examples:
The boring movie was very soporific.
The medicine had a soporific effect on me.
The teacher's voice was soporific, and I almost fell asleep.
The soporific music made everyone in the room yawn.
The soporific book put me to sleep in five minutes.
The soporific lecture made the students nod off.