Albert Barnes Commentary Lamentations 2:19

"Arise, cry out in the night, at the beginning of the watches; Pour out thy heart like water before the face of the Lord: Lift up thy hands toward him for the life of thy young children, that faint for hunger at the head of every street." — Lamentations 2:19 (ASV)

In - (or at) the beginning of the watches, "At the beginning of each night-watch" means all the night through. The Hebrews divided the night into three watches.