研究发现，睡眠时间与学习效率之间存在明显的相关性。

The study found a significant ___correlation between sleep duration and learning efficiency.

研究发现睡眠时长与工作效率之间存在显著的相关性。

The research shows a significant ___correlation between sleep duration and work efficiency.

研究发现睡眠质量与工作效率之间存在显著相关性。

Research has found a strong ___correlation between sleep quality and work efficiency.