As a correlate of psychological well-being, insomnia continues to be regarded as a critical indicator of both mental health and quality of life, appearing as a prominent diagnostic feature in both DSM-IV1 and ICD-102 (particularly in relation to depression and anxiety states), and included in many of the most commonly used health outcome measures, including the Nottingham Health Profile and the SF-364. The symptom of disturbed sleep is also included in most of the available schedules for assessing the mental health of elderly people, including the GMS, CAMDEX, and the CARE. Whether specified as a diagnostic criterion or included in a diagnostic algorithm, the assumption is made that certain patterns of sleep disturbance are associated with, and consequently predict, certain types of mental illness. The evidence would suggest, however, that the strength of the association, and hence the validity of this assumption, may diminish across the human lifespan.