INDIVIDUAL-DIFFERENCES IN STAGE-3 AND STAGE-4 SLEEP

Individual differences in stages 3 and 4 sleep are usually unrecognized, although several reports have mentioned large inter-individual variation of the stages within a restricted age range. This study investigated individual differences and nightly reliability in stages 3 and 4 sleep in a homogeneous sample of 34 young adult males. Sleep was analyzed from the final two of three consecutive lab nights. Results indicated that stage 4 sleep was an exceedingly prominent individual difference. Stage 4 occupied 7.9 to 50.0 percent of the first 3 hrs after sleep onset and was highly reliable across nights (.78). Individual differences were less pronounced in the second 3 hrs after sleep onset. Stage 3 and stage 4 were reliably distinguished. Persons high in one of these stages had relatively little of the other. Combining stages 3 and stage 4 may be a reasonable strategy if visually scored delta activity per se is of interest, but it may obscure potentially meaningful individual differences in which state is characteristically achieved.