אורח חיים רל״חOrach Chayim 238

Se’if 1א׳

צָרִיךְ לִזָּהֵר בְּלִמּוּד הַלַּיְלָה יוֹתֵר מִבְּשֶׁל יוֹם, וְהַמְבַטְּלוֹ, עָנְשׁוֹ מְרֻבֶּה.

Establishing fixed times for Torah study at night. This chapter contains two paragraphs: One must be more careful with nighttime Torah study than with daytime study, and one who neglects it receives a greater punishment.

Se’if 2ב׳

אִם יֵשׁ לוֹ חֹק קָבוּעַ לִלְמֹד כָּךְ וְכָךְ לְיוֹם וְהָיָה טָרוּד בַּיּוֹם וְלֹא הִשְׁלִימוֹ, יַשְׁלִימֶנּוּ בַּלַּיְלָה מִיָּד.

If a person has a fixed daily portion of study and was preoccupied during the day and did not complete it, he should make it up immediately at night.