Is it permissible to learn Torah She’Bchtav at night if one perfers to?
The Midrash (Yalkut Ki Tisa) asks, during the time Moshe Rabenu spent studying the Torah on Mount Sinai, how did he know day from night? And the Midrash says that when he was taught the written torah he knew it was day and when he was taught the oral torah, he knew it was night. From this Midrash different authorities learn that one should not learn the written Torah at night. The Poskim do conclude that there is no real prohibition on that and if one doesn’t know anything else or for some reason he need to study Torah at night he may do so. (Yabia Omer OC 17)