Subjects who took a nap regardless of the frequency and duration were one third less likely to die from heart attack or stroke than those who did not, the researchers reported in the February 12, 2007 issue of The Archives of Internal Medicine.
But some experts quickly warned that people should not rely on taking a nap to reduce their risk of heart disease…” (foodconsumer.org)
I am not sure if the napping is the independent variable here. It may be that people who take naps take better care of themselves in general, are less driven and more relaxed, etc. Correlation is not causation.