I have enlarged masseter muscles, can clenching at night be a cause? can it cause them to spasm and cause pain?

Yes, yes. : Physical therapy, splint therapy, Botox , are all effective ways to address TMJ pain. Placing your tongue on the roof of your mouth to keep you from clenching during awake hours can help minimize problems. But this won't solve problems when sleeping. You can risk damage to your teeth and the disc that the condyle rides on by ignoring it.