Does the “Sleep Effect” on Memory Depend on Sleep or on Night Time?

I is a well-established fact that recall of verbal information from long-term memory is superior after retention intervals of nocturnal sleep in comparison to retention intervals of daytime wakefulness (1-9). This finding was subsequently termed “sleep effect” (10). However, due to the confounding of the two factors sleep/wake and circadian phase that is inherent in the experimental design used, the effect cannot be attributed unambiguously to the independent variable sleep. This leaves the question whether the beneficial effect on memory is mediated by sleep per se or whether it “may actually be due to circadian variables which simply share the same period of time” (6, p. 372). To disentangle both factors, the sleep and the wake condition have to occupy the same nocturnal period. Though primarily aimed at isolating the differential effects of slow wave sleep (SWS) and rapid eye movement (REM) sleep, partial evidence for a beneficial effect of sleep per se comes from research investigating the first and second half of a 7 h nocturnal sleep separately. With the exception of Wagner et al. (11) all studies in this area found significantly better retention when subjects slept through the first 3-4 h of the night than Does the “Sleep Effect” on Memory Depend on Sleep or on Night Time?