Working shifts between 2100 h and 0700 h at a young age could increase the risk of developing multiple sclerosis (MS) later in life, according to two large case–control studies in Sweden. Among individuals who had worked shifts for at least 3 years before the age of 20 years, the odds ratio of developing MS was around 2.0 compared with those who had not worked shifts. Circadian disruption and restricted sleep, which modulate the immune system, might underlie the study results.