You may be looking forward to spring, but many people with multiple sclerosis are not—changing seasons can bring on a relapse. Scientists have attributed the seasonal fluctuations of MS to the rise and fall of vitamin D, which has anti-inflammatory properties and is produced by exposure to sunlight. Some studies, however, find that relapses increase in the spring and summer, when vitamin D levels are expected to be high, pointing to the possibility that other factors are involved. A recent study, published in Cell, suggests that melatonin, a hormone that regulates sleep cycles and the internal body clock, also plays a role.

Melatonin levels peak in the body during the darkest months. Mauricio Farez, a neuroscientist at the Raúl Carrea Institute for Neurological Research in Argentina, and his colleagues assessed a group of 139 MS patients in Buenos Aires and found a 32 percent reduction in the number of relapses in the fall and winter, when melatonin is high, compared with summer and spring.

To confirm melatonin's protective effect in the laboratory, the team gave daily injections of the hormone to mice with autoimmune encephalomyelitis, a widely used animal model of MS. It worked—the animals showed reduced clinical symptoms and a restored balance of T cells, white blood cells that contribute to a well-functioning immune system. Melatonin reduced the number of the T cells that promote inflammation while increasing regulatory T cells, defensive bodies that keep the immune system in check. The researchers observed comparable effects of the hormone on human immune cells grown in the lab.

The results show that melatonin regulates a key immune response, a fact that may be of use in other autoimmune diseases, particularly ones in which seasonal flare-ups occur, such as lupus and rheumatoid arthritis, says study co-author Francisco Quintana, an immunologist at Brigham and Women's Hospital.

“Our data show that melatonin might be one factor explaining the seasonal occurrence of relapses, in addition to infection and vitamin D,” Farez says, “and we need to see how they work together to fully understand their effects.” Farez's group is in the early stages of developing a clinical trial to test melatonin's effects in patients. Until those results come in, the researchers emphasize that no one should take melatonin to mitigate their MS symptoms because it may have unknown and possibly dangerous side effects.

Complex Origins

Studies have linked several environmental factors with the origin and progression of multiple sclerosis:

  • Smoking after being diagnosed with MS may accelerate disease progression, but quitting can slow the degeneration.
  • High salt intake may exacerbate MS disease activity and the risk of developing new brain lesions, according to a 2014 study.
  • Epstein-Barr virus may trigger MS and accelerate the progression of the disease, perhaps by activating the immune system.