Research has shown that sleep is connected to memory. This article discusses research about that topic. I don't have access to the original research study, only the summary article provided in the link. The study setup had participants learn made-up words prior to a night of sleep or the same amount of time awake. The participants were then asked to recall the words after a period of sleep or wakefulness.
The results showed that when compared to wakefulness, sleep helped the participants recall forgotten words. One of the researchers concluded that "sleep almost doubles our chances of remembering previously unrecalled material. The post-sleep boost in memory accessibility may indicate that some memories are sharpened overnight. This supports the notion that, while asleep, we actively rehearse information flagged as important."