Forgetting is a fundamental property of memory, but individuals vary in how quickly they forget. Sleep plays a crucial role in memory consolidation, particularly for fragile or weakly encoded memories. This study investigated whether long-term sleep quality predicts forgetting and whether this relationship is moderated by encoding strength. Fifty-one young adults (aged 18–32) watched two brief educational videos, then completed a cued recall task immediately and after a 1-, 2-, or 5-day delay. Half of the learning items were presented once (weak encoding) and half twice (strong encoding). Long-term sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), and forgetting was operationalized as the proportional decline in recall accuracy across sessions (Δ score). Linear regressions and mixed-effects models tested whether PSQI scores predicted forgetting, controlling for short-term sleep deviations. Results showed that poorer sleep quality significantly predicted greater forgetting, but only for weakly encoded items after a 5-day delay. No significant effects were found for strong encoding, shorter delays, or chronotype. The interaction between sleep quality and encoding strength was not significant. These findings suggest that poor habitual sleep might selectively impair long-term retention of weak memories, aligning with theories of selective sleep-based consolidation. However, several limitations, including fixed item assignments and small subgroup sizes, warrant cautious interpretation. This study adds nuance to forgetting curve research by highlighting how trait-level sleep differences may shape memory retention.