In lines 410-423 of Shelley's poem "Triumph of Life," Shelley is examining the light of earth which is important to survival. For one to end the day in the same way they began it is important. Beginning a day with the light of hope allows one to have a positive outlook on what is to come. Ending a day with the same light allows one to embrace the promise of hope again.
Shelley also refers to a dream, a lament (an expression of grief). within a dream, one loses the light one needs to see the promise of hope: the "light of heaven, whose half-extinguished beam" fails to provide the comfort of light one sees when awake.
In the end, Shelley is stating that it is the light which people seek so that they never become lost.