Does Napping Enhance the Consolidation of Clinically Relevant Information? A Comparison of Individuals with Low and Elevated Depressive Symptoms

Abstract

Purpose: Sleep, both overnight and daytime naps, can facilitate the consolidation of declarative memories in healthy humans. However, it is unclear whether such beneficial effects of sleep occur in special populations, such as individuals with elevated neuropsychiatric symptoms, and if they apply to clinically relevant material that may have personal significance to those populations.

Methods: We examined memory retention over a 60-minute interval of wakefulness or nap opportunity in participants with low or elevated scores (≤13 and ≥21, respectively) on the Beck Depression Inventory-II (BDI-II). Memory for depression-related information was assessed by (a) free-recall of a video depicting a personal experience narrative of the impact of depression on cognition and workplace performance; and (b) a paired-associates task linking depression-related cognitive symptoms to appropriate coping strategies.

Results: The results showed no overall difference in recall between the nap and waking condition. However, across the full sample of participants, there were significant positive correlations between total sleep time and paired associates recall, and slow wave sleep (SWS) percentage and story free recall performance. Unexpectedly, participants with elevated BDI-II scores exhibited better free-recall performance compared to those with low scores.

Conclusion: These results suggest that sleep, specifically SWS, may stabilize memories for clinically relevant information in populations with low and elevated depressive symptoms. The superior recall in participants with elevated-BDI scores may be related to the personal significance and stronger encoding of depression-related information. These observations raise the possibility that mnemonic deficits in depressed patients may be, at least in part, related to the type of information used to assess memory performance.

Keywords: consolidation; depression; memory; napping; sleep; slow-wave sleep.

Figure 1

Recall performance (mean scores out of 42 and 10 in (A and B), respectively) in participants with low (≤13) and elevated (≥21) scores on the Beck Depression Inventory-II (BDI) before and after the experimental condition (Nap vs Wake). (A) For the story free recall test, recall scores declined from pre- to post-condition testing for all participants. Further, the elevated BDI-group had higher recall scores than the low-BDI group. (B) For the paired associates test, recall performance also declined over time (pre- to post-condition). Error bars are omitted for clarity; standard deviations and group sizes are reported in Table 4.

Figure 2

Memory stability over time (calculated as pre-condition scores minus post-condition scores; higher values indicate greater memory decay, negative scores indicate enhanced recall over time) in relation to total sleep time (TST) in seconds (s) over the 60-min sleep opportunity. (A) There was a small (non-significant) negative correlation between TST and memory decay for the story free recall test. (B) For the paired associates test, there was a significant (p = 0.02) negative correlation between memory decay and TST, such that longer TST was associated with reduced memory loss.

Figure 3

Memory stability over time (calculated as pre-condition scores minus post-condition scores; higher values indicate greater memory decay, negative scores indicate enhanced recall over time) in relation to the percent of total sleep time spent in slow wave sleep (% SWS; n = 19). (A) There was a significant (p = 0.02) negative correlation between % SWS and memory decay for the story free recall test, such that greater % SWS was associated with reduced memory loss. (B) For the paired associates test, the correlation between memory decay and % SWS was negative without reaching statistical significance.