"People who spend their evenings in relatively bright light run the risk of stressing their bodies by ratcheting down the production of melatonin. Produced in the brain's pineal gland, this hormone plays a pivotal role in setting the body's biological clock – and, potentially, in limiting the development of certain cancers.
More than 100 young adults volunteered for a roughly 10-day research trial during which each took turns living in a light-controlled room at Brigham and Women's Hospital in Boston. From midnight until 8 a.m. the room was totally dark. At other times, researchers from Harvard Medical School tinkered with the room's lighting."