This chapter discusses whether shift work might be particularly harmful to men and women over 55 years of age. The essence of the problem for seniors who are shift workers therefore revolves around three interrelated concepts: age-related changes in the ability of the circadian system to phase adjust; age-related changes in the ability of the individual to stay awake and work in the late evening and overnight hours; and age-related changes in the ability of the individual to sleep at "unusual" times. Conventional wisdom asserts that seniors have flatter circadian rhythms, rendering the difference between day and night in their physiology less different for them than it is for younger adults. In addition to disorders of sleep and wakefulness, there appear to be both physical health and mental health consequences of shift work exposure. Social factors may also serve to render coping with shift work difficult.