There really are "morning" people and "night" people, and a new study of daily body rhythms suggests that when a married couple includes one of each, look out.

Whether you are most mentally alert in the morning or evening seems to depend on when your body temperature is highest, past researchers have found.Couples that are mismatched by time of maximum alertness tend to spend much less time together, fight more often and report lower levels of marital satisfaction, according to a study that appeared in the Journal of Marital and Family Therapy.

The good news, researchers from Brigham Young University and the University of Nebraska reported, is that in working out daily-cycle differences, many "morning and night" couples develop better communication and problem-solving skills than couples whose daily cycles are in sync.