When I applied mine heart to know wisdom, and to see the business that is done upon the earth: (for also there is that neither day nor night seeth sleep with his eyes:)

Wesley's Notes

8:16 To see - To observe mens various designs and employments, and their unwearied labours about worldly things. For there is - Having now mentioned the business which is done, or which man doth, upon earth, he further adds, as an evidence of man's eagerness in pursuing his business, for even by day and by night he (the busy man) seeth not sleep with his eyes. He grudges himself necessary refreshments, and disquiets himself with endless cares and labours.

Jamieson-Fausset-Brown Bible Commentary

16. Reply to Ec 8:14, 15. When I applied myself to observe man's toils after happiness (some of them so incessant as not to allow sufficient time for "sleep"), then (Ec 8:17, the apodosis) I saw that man cannot find out (the reason of) God's inscrutable dealings with the "just" and with the "wicked" here (Ec 8:14; Ec 3:11; Job 5:9; Ro 11:33); his duty is to acquiesce in them as good, because they are God's, though he sees not all the reasons for them (Ps 73:16). It is enough to know "the righteous are in God's hand" (Ec 9:1). "Over wise" (Ec 7:16); that is, Speculations above what is written are vain.

Matthew Henry's Concise Commentary

8:14-17 Faith alone can establish the heart in this mixed scene, where the righteous often suffer, and the wicked prosper. Solomon commended joy, and holy security of mind, arising from confidence in God, because a man has no better thing under the sun, though a good man has much better things above the sun, than soberly and thankfully to use the things of this life according to his rank. He would not have us try to give a reason for what God does. But, leaving the Lord to clear up all difficulties in his own time, we may cheerfully enjoy the comforts, and bear up under the trials of life; while peace of conscience and joy in the Holy Ghost will abide in us through all outward changes, and when flesh and heart shall fail.