According to a 2007 study on zebra fish, Melotonin may inhibit memory formation at night.  This is important for a language learner to know when to study long vocabulary lists.

“It was tremendous – the results were, excuse the expression, like night and day. We saw dramatic improvements in nighttime memory formation by inhibiting melatonin signaling, indicating that the reason the zebrafish did not form memories at night was because of the melatonin hormone.”