Polysomnograms were recorded for three consecutive nights on 14 male students (mean age, 16), comprising 4 identical and 3 fraternal twin pairs. The number of body movements and the measures related to the REM cycle, which correlated among the identical twins without reference to the physical parameters, were considered to be determined by a genetic trait. The sleep spindle density was in almost complete concordance between the identical twins and was associated with some physical parameters, which suggests that sleep spindles rather reflect the individual development. %S3, %SREM and the number of SREM showed a relationship to the physical parameters, without heritability. Twitch movements during REM sleep correlated not only between the identical twins but also between the fraternal twins.