Is REM latency a dying concept?

Twenty years have passed since the discovery that a shorter REM latency was one of the most significant biological correlates of depression. Its diagnostic and clinical discriminant power have been challenged both in terms of sensitivity and specificity. The present paper is a review of the relevant literature of these years until the present time and has focused on (a) technical problems concerning the REM latency, (b) its diagnostic value, (c) the state or trait meaning, (d) the use of the cholinergic and aminergic challenge, (e) the effects of treatments. A summary of (f) the effects of age and (g) REM sleep propensity as a cycle phenomenon have been added. The conclusions tend toward considering REM sleep disorders as an expression of a multiple circadian cycle abnormality, globally seen as a trait, itself highly correlated with psychiatric pathology and especially with affective disorders.