Rashi: Now, You shortened those years and brought them to be few years, like a sleep. The years for generations are 70 - "Yemei Shenoseinu Vahem Shiv'im Shanah" (verse 10), which are like a sleep - "b'Shuv Hashem Es Shivas Tziyon Hayinu k'Cholmim" (126:1). That refers to the 70 years of Galus Bavel. "Zeramtam" is an expression of gushing, like "Zerem Mayim" (Chabakuk 3:10).

Radak: You made man's years like a gush of water - it goes quickly. Alternatively, it is like a dream in Shenah (sleep).

Malbim: You tell people 'your 1000 years gushed (passed quickly), like yesterday gushed. It depicts the fleeting of time like gushing water; each current washes away its colleague. So each moment pushes the previous moment from the present to the past. "Shenah" corresponds to "v'Ashmurah va'Laylah." Just like there is no motion or action at night, for living beings sleep, so all 1000 years are as if you slept. There are two reasons why it is as if the 1000 years never were. (a) They passed from existence, like yesterday. (b) Even at the time, it is as if you slept then; you did not do anything.

Rashi: One who is born at night, he dies in the morning, at the end of the sleep.

Radak: [Man's life] is like grass, which withers and passes (dies) quickly, even in the morning. I.e. one dies in youth, like grass that is moist before the sun comes. In the morning, when the sun comes, it dries.

Malbim: You should view the 1000 years like morning, like grass withers, and like yesterday that passes. The coming morning, in the present, is like new vegetation that sprouts; the 1000 years that passed are like one day (yesterday).