Shulchan Arukh, Orach Chayim 238/שולחן ערוך·אורח חיים רל״ח

1Establishing 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.

2If 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.