There is an article in the August edition of the Journal of Clinical Sleep Medicine on napping, nighttime sleep, and cardiovascular risk factors in mid-life adults.
The authors measured sleep quantities over 10 nights with a combination of at-home and in-lab testing. They found that adults who nap more often had less nighttime sleep - this makes sense, as your body only needs so much sleep per 24 hour period. The authors also noted that adults who nap more had greater self-reported daytime sleepiness, fatigue, and bodily pain. This makes sense, too, as people who are more tired or in pain might nap more. These results support practicing good sleep hygiene in those patients with insomnia, because eliminating napping can improve nighttime sleep quantity and quality. I tell patients that napping in the daytime will usually take sleep away from them at night.
Finally, the authors noted that adults who napped more frequently had larger waist lines and a bigger body-mass index (BMI). Blood pressure level was not affected by the amount of napping. Therefore, in this study, napping was associated with an increase in some of the cardiovascular risk factors (BMI and waist circumference), but not blood pressure.