The fifth son of Leah is Issachar, and the Targum Pseudo-Jonathan on Genesis 30:18 gives his name a remarkable explanation.
Leah says, The Lord hath given me my reward, for that I gave my handmaid to my husband; even so shall his children receive a good reward, because they will occupy themselves with the law.
Issachar's name means "there is reward" — from yesh sachar. Leah is saying two things at once. She received a reward for giving Zilpah as a surrogate. And the tribe that will come from this son will receive a reward for a very specific reason: because they will occupy themselves with the law.
The Targum is identifying Issachar, at the moment of his naming, as the tribe of Torah scholarship. This is confirmed throughout the Hebrew Bible. The blessing of Moses will call the tribe of Issachar those who dwell in tents — the tents of Torah study (Deuteronomy 33:18). The book of Chronicles will describe the men of Issachar as those who had understanding of the times, to know what Israel ought to do (1 Chronicles 12:32).
There is a classical partnership in rabbinic literature: Zebulun and Issachar. Zebulun, a tribe of merchants on the coast, would support Issachar, a tribe of scholars in the interior. Zebulun did the trade. Issachar did the learning. Both shared the reward. The pattern is the root of every Jewish community's support for its yeshivot.
Leah foresaw all of it. Her son was conceived on a night purchased with mandrakes, and she knew at the naming that the reward would not be wealth or military might. It would be the Torah itself.
The takeaway: every tribe of Israel has a vocation, and the vocation is often spoken at the naming. Issachar was named for the law he would study.