They are summarized in three parts; evidence of intrusive memories; avoidance of associated traumatic stimuli and symptoms of increased arousal (van der Kolk, 2004). Symptoms may be reexperienced days or months after the event triggered by sounds, sights, smells, or unexpectedly without warning. Intrusive memories may be experienced as flashbacks or nightmares, and the person may then begin to avoid things that remind them of the event, such as approaching an "accident scene." In hyper-arousal the person may appear nervous, irritable or hypervigilant and this in turn can affect sleep or concentration. People suffering from post-traumatic stress disorder often present to their GP with physical symptoms such as headache, tiredness and chest pain and in some cases there is increased drinking or smoking habits.