Before dawn comes, the darkness of night thickens, and it is when daybreak is imminent that a heavy slumber descends upon one. It is then that one needs to gather strength, so that one will not fall asleep. The light [of the Messianic Era] is about to come, thank G-d; daybreak is beginning. Now is therefore the time when we should invest every effort and exertion to ensure that we are not overtaken by sleep.
(Rabbi Yosef Yitzchok Schneersohn in Likkutei Diburim)