If you speak to Luv, the owner of the Infusion Shop, at night, you will learn that he is unable to sleep because his baby keeps him up every night. It used to go to sleep easily, but recently its rattle was stolen by a bird.
The rattle can be found on top of the windmill in the center of Skyloft. However, it is impossible to reach this location without first completing the Lanayru Desert Silent Realm and receiving the Clawshots.
Go to the entrance to the Waterfall Cave and clawshot to the vines on the side of the islands above you. After reaching the island with the large waterfall, go to the far bank. Jump off and skydive very far forward into the nest above the windmill. Pull out your Gust Bellows to clear the dust away from the rattle.
Return to Luv at night and give him the rattle. He will thank you and give you 5 Gratitude Crystals.