Obstructive sleep apnea (OSA) is a common disorder that remains under-treated due few therapeutic options beyond continuous positive airway pressure (CPAP). In patients with OSA, upper airway obstruction is caused by collapse of pharyngeal structures during sleep. It is known that mouth breathing increases upper-airway collapsibility during sleep and may contribute to the occurrence of sleep disordered breathing. In addition, it was shown that patients with a high percentage of mouth breathing during sleep were less adherent to CPAP therapy. Therefore, one potential solution to these problems is to use Varnum's mouthpiece to prevent mouth breathing during sleep.