Regularly working second or third shift can increase the risk of gastrointestinal (GI) troubles and late-pregnancy miscarriage, according to two separate studies. In a study of 343 U.S. auto factory workers, researchers found that working the second shift (typically 2:30 to 11 p.m.) was associated with more GI symptoms and diagnoses than working the first (day) shift. No one in the study worked the third (night) shift.

The researchers concluded that two circumstances-second-shift work and widely varying work start and end times-may increase the risk of GI problems, including nausea, indigestion, abdominal pain, loss of appetite, and diarrhea. Second-shift workers were almost three times as likely as first-shift (day) workers to report receiving a diagnosis for a GI condition, such as stomach ulcer, ulcerative colitis, or colon polyps.

The second study involved over 41,000 pregnancies among Danish shift workers. Researchers found overall, only about 1% of the pregnancies studied ended in miscarriage or stillbirth. But 11 of the 420 women who regularly worked night shift (2.6%) experienced a miscarriage late in pregnancy or a stillbirth.

When they considered other factors, such as age, smoking habits, and physical demands at work, they found that women who typically worked night shift had an 85% higher risk of losing a fetus late in pregnancy. Working other shifts, including shifts that require some night work, wasn't associated with miscarriage.

Researchers suspect that shift work throws off the body's internal clock and rhythms and may cause changes in blood pressure and hormone production, triggering problems.

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