ויהי בבקר והנה היא לאה

Aside from the practical issues of how could Yaakov have not realized who he was sleeping with there is a halachic issue here. The Gemara in Nedarim (20b) has a list of פגומים (who the Gemara calls rebellious), one of them is בני תמורה, which the Rishonim explain to mean that he has 2 wives and thinks he is having relations with 1 and is actually having relations with the other. This would seem to be exactly the case of Yaakov and Leah, he thought that he was sleeping with Rachel not Leah and Chazal state that Reuven was born from Yaakov's first emission (ראשית אוני). it is very hard to believe that ראובן one of the שבטים, would be considered פגום.

The Raavad is bothered with this and answers that Yaakov was before Matan Torah and was not מקפיד on this (clearly very difficult). He offers another answer that Leah did not get pregnant that night. However, this contradicts the Chazal about Reuven being from Yaakov's first emission. See the Magen Avraham ( סי ר"מ סעיף ט) for another difficult answer.