I’ve written in the past on this topic, which is controversial in the literature for both adults and children. With parafunctions such as bruxism, there isn’t any way to engage in research based study because no two subjects have the same characteristics, bite or other factors considered to be pre-disposing to bruxism.
In children and adults, there are known relationships between sleep disordered breathing/TMJ damage and clenching or bruxing; however, there is no way to have a “double blind” study to come to a definitive reason “why do people clench or brux their teeth”.
Contracting the jaw muscles, whether by clenching or bruxing is known to “open the airway” in sleep apnea and often subsides when the apnea is treated. Likewise, a damaged/misaligned TMJ can contribute to grinding/clenching.
Over my 30 years of treating TMJ and Sleep apnea, I have seen the majority of patients who had initial complaints of bruxing/clenching and who were properly treated with orthopedic orthotics had a lessening or complete resolution of the bruxing. Many of my colleagues who exclusively treat TMJ also see this response.
Dental nightguards sometimes lessen clenching or grinding, but more often exacerbate it. A dental nightguard has a different design and mechanical force on the TMJ than an orthopedic TMJ appliance. If you find you are clenching or grinding harder with your dental nightguard that would strongly suggest an underlying sleep breathing problem or TMJ damage that is being exacerbated.
If your current appliance is causing more problems with clenching/grinding, consider having a TMJ specialist evaluate the jaw joints, airway, and the design of your dental appliance to assess if it might be contributing to increased joint pressure or airway constriction, which could be part of the problem.