Human existence is deeply fragile and fleeting, especially when contrasted with the eternal nature of God. The span of a person's days passes with the suddenness of a rushing flood, an elusive dream, or a blade of grass that sprouts and quickly dies. God sweeps away the years of human life with immense speed, moving like a powerful torrent of water. Because time rushes by so quickly, the human experience is often compared to a state of sleep. The primary approach among commentators is that just as a person wakes up to realize a dream lacks physical reality, human life itself ultimately appears insubstantial [רד״ק, מצודת דוד, ביאור שטיינזלץ]. Furthermore, time slips away without a person even noticing, much like the lack of awareness experienced while sleeping [מאירי]. This reflects the reality that God limited human life to a mere seventy years, a period that flies by and feels like nothing more than a brief slumber [רש״י].
Alternatively, this sleep can represent death itself, acting as an eternal rest [אבן עזרא]. In a different light, the realization of life's brevity can be seen as a divine kindness. It is meant to instill a healthy fear of mortality, motivating a person to repent while still alive. By doing so, the end of their life is not a tragic loss, but rather the peaceful sleep of the righteous [אלשיך].
When a person awakes in the morning, they find that their life mimics the natural cycle of field grass. The morning represents the days of youth, a time of blossoming, renewal, and gathering strength [מצודת ציון, מצודת דוד, אלשיך]. Yet, this vibrant blossoming is painfully brief. Just as green grass flourishes in the morning dew only to wither and dry up quickly under the harsh heat of the sun, human days fade with the same rapid finality [מאירי, ביאור שטיינזלץ]. Time constantly shifts, and every passing day consumes a little more of a person's vitality. By the time the evening of life arrives, their strength is entirely depleted, and the present moment fades into a past that is gone forever [מלבי״ם].