Some science behind the scenes
Jaw clenching is a side effect of spiritual experience. The correct name for jaw clenching that is involuntary is Trismus.
It can be caused by a number of diseases, for example Pericoronitis (inflammation of soft tissue around impacted third molar) ; inflammation of the muscles of mastication; and peritonsillar abscess, a complication of tonsillitis. But in this context we are interested in jaw clenching that is not a result of illness.
Trismus is a common temporary side effect of many stimulants of the sympathetic nervous system. Users of drugs like methylenedioxymethamphetamine (MDMA), methamphetamine (meth), methylphenidate (Ritalin), mephedrone (4-MMC), and amphetamine for example, get jaw clenching.
But, jaw clenching can also occur to people who do not take drugs.
One of the signs of jaw clench is ‘teeth grinding’ - Bruxism (from the Greek (brugmós), "gnashing of teeth"). Bruxism is typically accompanied by the clenching of the jaw and can even occur during short naps.
Given the vividness of dreams once we have established a spiritual pattern, this may thus be a common side effect of the techniques of dreaming and lucid dreaming. It can also occur as an involuntary side effect during relaxation.