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Sleep consists of several cycles, from lighter to deeper sleep (stages 1 though 4), REM, and transitioning back to lighter sleep. The frequency and duration of cycles varies. Jaw muscle activity generally occurs in transitional stages (it is a normal event), from stage 2 into deeper stage 3, and in REM, as deep sleep transitions into lighter stages. The final transition of sleep, from stage 1 to being awake, varies the most. Waking to an alarm shortens it or avoids it altogether, while "sleeping in" allows for the longest duration. Jaw muscle activity during this time plays a role in the intensity of morning headache, which may explain the common report of "weekend headache".
The orientation of the mandible, the duration of muscular activity, the frequency of activity, and most importantly, the intensity of contraction dictates the degree of signs and symptoms.
The orientation of the mandible is a function of the muscles of mastication. A centered, balance position (2) results in temporal headache. When the jaw is protruded (a function of the Lateral Pterygoids) and clenched there, symptoms are reported in the face and sinuses, due to the strain on the pterygoid plates, and in the neck, due to the obligatory balancing of the skull by the trapezius (3). An excursive clench (4) strains the TMJ. The orientation of the mandible created by the lateral pterygoid and the occlusal scheme dictate the degree, if any, of joint derangement.