When you are suffering from sleep apnea and other sleep-related breathing disorders during your sleep, you need an overnight sleep study. Especially patients who need adjustments to their continuous positive airway pressure (CPAP) devices, patients with possible cases of narcolepsy who have a hard time staying awake during the day, and patients who feel abnormally fatigued during the day even though they sleep throughout the night can also be part of an overnight sleep study.