WordNet sense 1 (sitting on eggs so as to hatch them by the warmth of the body):

incubation

WordNet sense 2 (persistent morbid meditation on a problem):

pensiveness

WordNet sense 3 (persistently or morbidly thoughtful):

broody, contemplative, meditative, musing, pensive, reflective, ruminative, pondering

WordNet sense 4 (exhibiting or characterized by careful thought):

thoughtful