Some patients have a sleep disorder called Obstructive Sleep Apnea (OSA) which at times can be easily identified by a physician by way of history alone. For these patients, when they fall asleep at night the muscles in their airway relax causing the airway to collapse on itself.
This is sometimes evident by snoring while sleeping and the patient being very fatigued in the daytime.
With a special device called CPAP (continuous positive airway pressure) and a nasal mask, air can be pushed into the patient's airway such that it is forced to stay open rather than collapse.
This results in the patient getting a better night's sleep, including better oxygenation and higher quality sleep, and also the patient can perform better during the day.