The age-related structural changes in sleep considered in the previous chapter were described as ‘direct’ in the sense that they were assumed to result directly from changes in the mechanisms which control sleep and waking. In addition to such directly influenced change, the process of ageing can also affect sleep indirectly as suggested in Figure 2.1. Conceptually indirect influences upon sleep may be divided into those which arise in the internal environment (inside the body), and those which arise in the external environment (outside the body). Both sources of influence will be considered in the present chapter, although each will be approached in a slightly different way. Internal influences are mainly organic or biological in origin and include not only the changes associated with so- called normal ageing, but also those arising from the illnesses and diseases which become more prevalent with increasing age. In the discussion which follows some typical internal changes will be identified and described and their likely influence upon the structure and quality of sleep will be considered.