Researchers have looked at the ends of chromosomes for the answer to aging, and they're measuring the effect of mediation on the health of the genetic material. At the end of the chromosomes are long strands of what scientists call "redundant" codes called telomeres. The telomeres are repeated genetic sequences that tell the proteins that they've reached the end of the line when the genes are being copied. Telomeres appear to get shorter as we age, and a recent paper suggested that their length may be tied to our stress levels.
The paper discusses the effect of "psychological stress cognitions, particularly appraisals of threat and ruminative thoughts," meaning stress and constantly thinking about it, on the telomere length in comparison to mindful meditation techniques, which shift thinking "from threat to challenge, decrease ruminative thought, and reduce stress arousal." They point out that several stress hormones, including cortisol, insulin, and oxidative stress have been linked to shortened telomeres, while androgens (building hormones), growth hormone, and proper nervous system tone (vagal, or parasympathetic- basically the opposite of the fight-or-flight response) are linked to preservation of the strands.
Science isn't sure exactly why, but shortened telomeres result in chromosomal damage, and that triggers the cells to start dying prematurely. If something as simple as being conscious of your thoughts can help prevent that, then by all means- take a breather today.
Elissa Epel, Jennifer Daubenmier, Judith Tedlie Moskowitz,Susan Folkman, and Elizabeth Blackburn (2009). "Can Meditation Slow Rate of Cellular Aging? Cognitive Stress, Mindfulness, and Telomeres." Longevity, Regeneration, and Optimal Health: Ann. N.Y. Acad. Sci. 1172: 34–53 (2009). doi: 10.1111/j.1749-6632.2009.04414.x