It is vain for you to rise up early, to sit up late (comp. Isaiah 5:11); i.e. to be "careful and troubled" about your work in the world, whatever it is. To eat the bread of sorrows. To feed, as it were, on sorrow—and trouble and care—to make your lives a burden to yourselves through your carefulness.
For so he giveth his beloved sleep; rather, surely he giveth to his beloved in sleep; i.e. in their sleep. The teaching is similar to that of Exodus 14:14; Isaiah 30:7, Isaiah 30:15; Matthew 6:25-34.
God gives to men that which he knows they have need of, if they have only the faith to "sit still" and "wait."