Selective slow-wave sleep (SWS) deprivation and SWS rebound: do we need a fixed SWS amount per night?

Visually scored delta activity (stages 3 and 4, SWS) as well as computerized delta activity measures increase after total and selective sleep deprivation. It is, however, still controversial if SWS amount is only a function of prior waking duration, or if it is related to the structure of the previous sleep period (i.e., to the time spent in SWS). In order to clarify if the amount of SWS is crucial in determining SWS recovery, we selectively deprived SWS during two nights to assess the presence of a compensatory SWS rebound in the following recovery night. Ten normal males slept for 6 consecutive nights in the laboratory. After an adaptation and two baseline nights (BSL; BSL-A), selective SWS deprivation was accomplished for two consecutive nights (DEP-1; DEP-2), by means of an acoustic stimulation technique. A recovery (REC) night then followed. An almost complete selective SWS suppression during both deprivation nights was achieved. A significant increase of S4 and SWS in the REC as compared to the BSL-A paralleled a significant shortening of S3 and S4 latencies. S2 percentage significantly increased during both DEP nights with respect to the other experimental nights. There was no significant difference among nights with regard to total sleep time, percentage of REM sleep, stage 1, movement time, number of awakenings and number of movement arousals, indicating that the acoustic stimulation technique did not dramatically disrupt normal sleep continuity and architecture. These results indicate that SWS rebound after selective SWS deprivation can be ascribed to the loss of SWS accumulated during two consecutive nights, further supporting the idea that the delta sleep amount is more linked to SWS in the previous sleep periods than to the total sleep duration.