Sleep supports processes necessary for hippocampus-dependent memory consolidation. While experimental paradigms for sleep-dependent memory consolidation commonly involve comparing sleep/wake delays at opposing diurnal phases, whether the memory benefit of sleep is influenced by circadian phase is unclear. Using forced desynchrony (FD), this study tested the hypothesis that the sleep benefit upon hippocampus-dependent memory consolidation depends on the circadian phase at which sleep occurs.
12 adolescents (6F, mean 13.6±0.8 years) completed seven cycles of 28-hour FD. Each 28-hour FD cycle included 17.5 hours scheduled wakefulness and 10.5 hours sleep, decoupling sleep timing from circadian phase. Participants trained on the hippocampus-dependent Mnemonic Similarity Task and tested approximately 12-hours later, in four conditions bracketing wake or sleep during either the biological day or night. Thus, both initial learning and delayed consolidation were tested at opposing levels of sleep pressure and circadian phase.
Repeated-measures ANOVA found no evidence that sleep pressure, circadian phase, or their interaction predicted initial encoding ability (all p’s > 0.05). When assessing within-subject change scores, a trend emerged indicating an interaction of sleep and circadian phase (F(1,11)=3.56; p=.086) affecting consolidation. Thus, when learning occurred in the circadian morning, a delay that included sleep was associated with reduced forgetting compared to a delay without sleep (t(11)=-1.83; p=.045 [one-tailed]). No benefit of sleep was present across the circadian night (t(11)=0.37; p=.72 [one-tailed]).
These results provide preliminary evidence that circadian processes may influence the sleep benefit for hippocampus-dependent memory consolidation: a sleep benefit occurred only across the circadian day. Memory-regulating oscillatory properties of sleep, such as sleep spindles, are sensitive to circadian influences. Our future analyses will examine sleep EEG during FD as one potential factor mediating these results. While these results are preliminary and from a small sample, they suggest a need for more direct consideration of circadian timing when assessing sleep-dependent memory consolidation.
Support (If Any)
R01DK101046 (MAC), K01MH109854 (JMS)