Sleep apnea is a serious sleep disorder, in which people :

The correct answer is C. Obstructive sleep apnea is a disorder in which the upper airway repeatedly collapses during sleep, so breathing briefly stops again and again, often dozens of times an hour. Each pause drops oxygen levels and forces the brain to wake the person briefly, which is why the sleep is fragmented and the patient feels tired through the day even after a full night in bed.

• A is wrong. Walking during sleep is sleepwalking (somnambulism), a parasomnia, and has nothing to do with breathing.
• B is wrong. Sleeping with half-open eyelids is nocturnal lagophthalmos, a condition linked to facial nerve or eyelid problems, not to sleep apnea.
• D is wrong, though it is the trap. Loud snoring is a common symptom of obstructive sleep apnea, but snoring alone does not define the disorder. The defining feature is the pause in breathing.

The core concept is the difference between a symptom and the disorder itself. For Mains, sleep apnea links to public health and occupational safety, since untreated patients have higher risks of hypertension, road accidents and poor productivity. Treatment includes CPAP therapy.

Trap: Choosing D because snoring is the most familiar sign, while ignoring that the question asks what the person actually does during sleep.