A night guard for sleep apnea, also known as an oral appliance or mandibular advancement device, is a custom-made dental device prescribed by a dentist to treat mild to moderate obstructive sleep apnea. It works by repositioning the lower jaw and tongue to keep the airway open during sleep, allowing for improved breathing and reduced snoring. It is a non-invasive and comfortable alternative to continuous positive airway pressure (CPAP) therapy, which is a common treatment for sleep apnea.