To understand why this scenario never happens, one must understand how spiders perceive their environment. House spiders do not wander rooms out of aimless curiosity. Common indoor species like cellar spiders (Pholcidae) and cobweb weavers (Theridiidae) rely heavily on vibration-sensitive hairs called trichobothria and microscopic sensory slits across their exoskeletons to read physical vibrations.
A sleeping human is an engine of violent sensory output to a 10-milligram arachnid. Inhaling and exhaling creates heavy rhythmic airflow. The cardiovascular system pumps blood, producing detectable pulse vibrations across mattresses and sheets. Involuntary muscle twitches and snoring sound to a spider like an ongoing seismic catastrophe. Approaching a human mouth offers zero survival reward; it represents a warm, humid cavern blasting carbon dioxide, signaling an apex predator.
