Teens who engage in shift work and disrupt their circadian rhythm may have an increased risk of multiple sclerosis, Swedish researchers say. Dr. Anna Karin Hedstrom and colleagues at the Karolinska Institutet in Stockholm analyzed data from two population-based studies -- one with 1,343 incident cases of MS and 2,900 controls, and another study with 5,129 prevalent MS cases and 4,509 controls. The team compared the occurrence of MS among study subjects exposed to shift work at various ages against those who had never been exposed. All study subjects lived in Sweden and were between the ages of 16-70. Shift work was defined as permanent or alternating working hours between 9 p.m. and 7 a.m., Hedstrom said. The study, published in the Annals of Neurology, found those who worked off-hour shifts for three years or longer before age 20 had a two-fold risk of developing MS compared with those who never worked shifts. The authors suggested disruption of circadian rhythm and sleep loss may play a role in the development of MS, but the exact mechanisms behind the increased risk remain unclear and further study is needed. \"Our analysis revealed a significant association between working shift at a young age and occurrence of MS,\" Hedstrom said in a statement. \"Given the association was observed in two independent studies strongly supports a true relationship between shift work and disease risk.\"