A sleep schedule incorporating naps benefits the transformation of hierarchical knowledge

Abstract

Study objectives: The learning brain establishes schemas (knowledge structures) that benefit subsequent learning. We investigated how sleep and having a schema might benefit initial learning followed by rearranged and expanded memoranda. We concurrently examined the contributions of sleep spindles and slow-wave sleep to learning outcomes.

Methods: Fifty-three adolescents were randomly assigned to an 8 h Nap schedule (6.5 h nocturnal sleep with a 90-minute daytime nap) or an 8 h No-Nap, nocturnal-only sleep schedule. The study spanned 14 nights, simulating successive school weeks. We utilized a transitive inference task involving hierarchically ordered faces. Initial learning to set up the schema was followed by rearrangement of the hierarchy (accommodation) and hierarchy expansion (assimilation). The expanded sequence was restudied. Recall of hierarchical knowledge was tested after initial learning and at multiple points for all subsequent phases. As a control, both groups underwent a No-schema condition where the hierarchy was introduced and modified without opportunity to set up a schema. Electroencephalography accompanied the multiple sleep opportunities.

Results: There were main effects of Nap schedule and Schema condition evidenced by superior recall of initial learning, reordered and expanded memoranda. Improved recall was consistently associated with higher fast spindle density but not slow-wave measures. This was true for both nocturnal sleep and daytime naps.

Conclusion: A sleep schedule incorporating regular nap opportunities compared to one that only had nocturnal sleep benefited building of robust and flexible schemas, facilitating recall of the subsequently rearranged and expanded structured knowledge. These benefits appear to be strongly associated with fast spindles.

Clinical trial registration: NCT04044885 (https://clinicaltrials.gov/ct2/show/NCT04044885).

Keywords: memory consolidation; memory reactivation; nap; prior knowledge; schema; sleep spindles.

Figure 1.

Study protocol and schema-learning sessions. The experiment consisted of two baseline adaptation nights and two sleep manipulation periods which included two recovery nights. Polysomnography was conducted during nights and naps on B2, M1₅, R1₁, M2₁, M2₃, and R2₁. The schema-learning paradigm consisted of four phases, each involving immediate and delayed test sessions: Initial learning (immediate test, 12-h test), Schema accommodation (immediate test: 12-h test, 24-h test), Schema assimilation (immediate test, 12-h test, 24-h test, 84-h test), and Schema restudy (immediate test, 12-h test, 72-h test).

Figure 2.

(A) (1) Phase 1a: Learning the schema to criterion. Participants performed learning blocks with active feedback (correct answer highlighted with green). Examples of test trials are shown for (2) phase 1 adjacent pairs, and (3) phase 1 inference pairs. (B) Schema-related memory integration and introduction of the No-schema condition. Participants were shown six alternating learning (with feedback) and test (with no feedback) blocks for the Schema condition. This was repeated for the No-schema condition. L, learning; T, Testing.

Figure 3.

(A) Schema accommodation (learning reordered material). In each condition, the position of four individuals in each hierarchy was changed. (B) Schema assimilation (additions to reordered material). In this phase, three novel items were added to each hierarchy.

Figure 4.

Behavioral results across all phases. Performance on immediate and delayed test sessions for the Nap and No-Nap groups are shown for both the Schema and No-schema conditions. **p < .01, *p < .05.

Figure 5.

Correlation between schema benefit and spindle density across multiple nap sessions. Higher schema-driven memory benefits after assimilation were associated with increased N2 fast spindle density across naps (M1₅, M2₁, M2₃).

Figure 6.

Correlation between schema benefit and spindle density across multiple nights of sleep. Higher schema-driven memory benefits after assimilation and restudy were associated with increased N2 fast spindle density across manipulation nights (M1₅, R1₁, M2₁, and R2₁).