A cloud of stress is looming over most Americans.  A survey conducted by NPR for the Robert Wood Johnson Foundation has found that almost half of respondents have had to deal with a stressful event sometime in the past year.  Twenty-six percent say they've been under "a great deal" of stress within the past month.  Among the top causes of major stress in the lives of Americans are health problems and loss of a loved one.  The most common symptom for those under pressure is lack of sleep.  Seventy percent report sleeping less than usual while worried about a personal problem.  Others say their stress manifests itself in lack of exercise and loss of appetite.