Rashi: He is like a worker who toils the entire day, and yearns when will come the shade of night. Malbim - a slave is not exempted from working when he finishes a fixed task, only when night comes.

Ibn Ezra: This is when he finished his work.

Rashi: He is like a worker who toils the entire day, and desires sunset.

Devar Eliyahu: The slave longs for night, when he rests. The worker hopes to receive the wage for his work.

Malbim: A worker longs to finish the job that he accepted - then he rests. Either way, when he is ill and idle from work, he cannot reach his perfection. If so, you cannot say that Hashem afflicts a Tzadik amidst love, to cleanse his sin. This distances him from his perfection!