Definitions of REM latency: some comparisons with particular reference to depression.

Unlike sleep stages, which are assessed by standardized criteria, an informal survey by us showed that the definition of REM latency is relatively idiosyncratic. Differences in definition arise for two reasons: Differences in the definition of sleep onset, and the inclusion or exclusion of wakefulness that may occur prior to the first REM period. Seven definitions are identified. Each was applied to 70 records taken from 14 depressed patients, 5 remitted depressives, and 5 normal subjects. In all three samples, REM latencies calculated on the basis of the different definitions differed significantly. Reasons for adopting a common definition are discussed. It is suggested that, if established, the reliability and validity (concurrent and predictive) of the available measures would provide an empirical basis for selection.