Depression in adults may have a number of complex and interacting symptoms. This variability poses a major challenge to researchers attempting to understand and treat the disorder. Typical symptoms exhibited by adult depression patients include, agitation, anxiety, extreme grief, insomnia and irritability. Some adults have one or more combination of medical illness that also can cause the onset of depression. Some emotional symptoms with life changes such as, retirement, loss, bereavement, grief, loneliness, moving house, disability, lack of exercise, increased insomnia and multiple illness may often trigger the onset of depression in adults. Stressful life events may contribute to recurrent episodes of depression in adults.