Bruxism is classified into two categories:
Primary (idiopathic) is when the disorder is not a related medical condition, or
Secondary (iatrogenic) is when side effects of medications, neurological or developmental disorders are causing the behavior.8
Primary bruxism is further divided into two types: awake and sleep bruxism. Clenching or grinding of the teeth is a common activity that can occur both during the day and at night.
Awake bruxism happens during the day with clenching being more prominent. It is defined as awareness of jaw clenching and appears to be semi-voluntary. It is usually correlated with high anxiety and stress. Awake bruxism is relatively common involving 20% of the adult population and 18% of children. It effects females more than males.1 Awake bruxism symptoms usually worsen throughout the day. Clenching during the day increases the risks of clenching or grinding at night.
Sleep bruxism occurs at night while sleeping with grinding being more prominent. It is defined as a sleep-related movement and is involuntary. It occurs in about 8%-10% of the population, with a lack of awareness from about 80% of the bruxers.; It effects both males and females equally.1,8 Sleep bruxism symptoms are usually worse in the morning, especially upon waking, and improve during the day.
One study reported sleep arousals induced sensory stimulation, which triggered episodes of sleep bruxism. Sleep arousals are a sudden change in the depth of the sleep stage and may also be accompanied by increased heart rate, respiratory changes and muscular activity, such as leg movements. It has been shown the majority (86%) of sleep bruxism episodes occur during periods of sleep arousal as a person goes from a deeper stage of sleep to a lighter stage of sleep.9
A typical sleep cycle is 90 to 110 minutes of sleep with three to five cycles per night. Sleep is determined by two categories: non-REM (quiet sleep) and REM (active sleep). There are four stages of sleep. Stages 1‑2 are light sleep and stages 3‑4 are deep sleep, with REM occurring in the third stage. A micro-arousal is a shift in sleep occurring in the deeper sleep. In this stage there are 3‑10 second rises in brain activity, heart rate and muscle tone. Micro-arousals occur 8‑15 times an hour, leaving bruxism secondary to these arousals. More destructive bruxers have greater amount of episodes and time in the REM stage.10