Disrupting the biological clock can result in blood sugar spikes and insulin resistance. Harvard/Brigham and Women's Hospital researcher Frank A.J.L. Scheer and colleagues found that much of the body's biological clock -- its circadian rhythm -- keeps day shift time even when a person goes on the night shift.

They have selected five women and five men who volunteered to undergo a kind of progressive jet lag. For eight "days," the participants ate and slept on a 28-hour schedule, during which they ate four identical-calorie meals. When their cycle shifted about 12 hours out of phase -- when they were sleeping during the day and up at night -- the participants' bodies got seriously out of rhythm:

After meals, three out of eight participants tested had blood sugar spikes and insulin resistance similar to those seen in people with diabetes or pre diabetes. The participants' bodies made more insulin, yet their blood sugar went up. Blood levels of leptin went down. Long term, this would increase obesity risk, as decreased leptin makes people burn less energy while craving more food. Cortisol - the so-called stress hormone that affects blood pressure and blood sugar -rose and dropped at the wrong time.

The study participants went back to their normal sleep schedules after the experiment. That's not an option for shift workers. Long term, the changes seen in the Scheer study may have ominous consequences. Animal studies, the researchers note, show that "internal desynchronization" from repeated shifts in the day/night cycle cause premature death.

Source: Proceedings of the National Academy of Sciences. Online edition, March 2, 2009