behavior), which leads to reduced muscle flexibility, tone, strength and endurance. This manifests as limited mouth opening and a sensation that the teeth are not fitting properly. Persons with TMD have a higher prevalence of psychological disorders than people without TMD. People with TMD have been shown to have higher levels of anxiety, depression, somatization and sleep deprivation, and these could be considered important risk factors for the development of TMD. In the 6 months before the onset, 50–70% of people with TMD report experiencing stressful life events (e.g. involving work, money, health or relationship loss). It has been postulated that such events induce anxiety and cause increased jaw muscle activity. Muscular hyperactivity has also been shown in people with TMD whilst taking examinations or watching horror films. Others argue that a link between muscular hyperactivity and TMD has not been convincingly demonstrated, and that emotional distress may be more of a consequence of pain rather than a cause. Bruxism Bruxism is an oral parafunctional activity where there is excessive clenching and grinding of the teeth. It can occur during sleep or whilst awake. The cause of bruxism itself is not completely understood, but psychosocial factors appear to be implicated in awake bruxism and dopaminergic dysfunction and other central nervous system mechanisms may be involved in sleep bruxism. If TMD pain and limitation of mandibular movement are greatest upon waking, and then slowly resolve throughout the day, this may indicate sleep bruxism. Conversely, awake bruxism tends to cause symptoms that slowly get worse throughout the day, and there may be no pain at all upon waking. The relationship of bruxism with TMD is debated. Many suggest that sleep bruxism can be a causative or contributory factor to pain symptoms in TMD. Indeed, the symptoms of TMD overlap with those of bruxism. Others suggest that there is no strong association between TMD and bruxism. A systematic review investigating the possible relationship concluded that when self-reported bruxism is used to diagnose bruxism, there is a positive association with TMD pain, and when more strict diagnostic criteria for bruxism are used, the association with TMD symptoms is much lower. Self-reported bruxism is probably a poor method of identifying bruxism. There are also very many people who grind their teeth and who do not develop TMD. Bruxism and other parafunctional activities may play a role in perpetuating symptoms in some cases. Other parafunctional habits