Airway and sleep are a growing part of dentistry, because the mouth and jaw have a lot to do with how well you breathe at night. For some people, a custom oral appliance is part of the answer.
A sleep appliance is a custom-fit device worn at night that gently postures your lower jaw forward to help open up the airway. Because it treats a medical condition, a prescription from a sleep doctor is required before one can be made, which means a sleep test comes first and is mandatory. That evaluation is the right starting point, and from there we can talk about whether an appliance fits your situation.
Mouth breathing at night, especially paired with snoring, is often a sign of an underlying airway issue like obstructive sleep apnea or upper airway resistance. A CPAP machine is still the gold standard for moderate to severe sleep apnea, but a lot of people can't tolerate one, and for them a custom oral appliance is the next best alternative.