Natsuhiboshi

is the title of the song that both Sumaru and Natsuhi sang in the Star Guard Mission. It is a lullaby Natsuhi sang to her son when he was an infant. He usually sings the lullaby whenever he remembers his deceased mother.

Rōmaji
Natsuhiboshi, naze akai? Yuube kanashii yume wo mita. Naite hanashita. Akai me yo.

Natsuhiboshi, naze mayou? Kieta warashi wo sagashiteru Dakara kanashii yume wo miru

Natsuhiboshi, naze akai? Yuube kanashii yume wo mita. Naite hanashita. Akai me yo

Natsuhiboshi, naze mayou? Kieta warashi wo sagashiteru Dakara kanashii yumewo miru

English
Summer Star, Why are you so red. Because, I had a sad dream last night. My eyes are red from the tears I've shed. Swollen as I cried.

Summer Star, Why've you lost your way? I'm searching for a child whose gone afar. He can't be found, though I've searched all day. My sad dreams come once more.

Summer Star, Why are you so red. Because, I had a sad dream last night. My eyes are red from the tears I've shed. Swollen as I cried.

Summer Star, Why've you lost your way? I'm searching for a child whose gone afar. He can't be found, though I've searched all day. My sad dreams come once more.

Trivia

 * may refer to the planet Mars which is also known as the "star in the summer".
 * It's been played in the background in several episodes during the filler arc "Past Arc: The Locus of Konoha" along with other original Naruto scores.
 * In Naruto: Shippūden episode 178, the song can be heard playing in the background during Iruka's flashback, before the episode intro.
 * In Naruto: Shippūden episode 181, the song can be heard playing in the background as Katazu gives his explanation as to why he has taken his twin brothers place.
 * In Naruto: Shippūden episode 186, the song can be heard playing in the background in about the middle of the episode while Naruto, Sakura and Ino talks about Lee's injury.
 * In Naruto: Shippūden episode 193, the song can be heard playing in the background near the end of the episode while Kisuke's ghost says goodbye to Naruto and asks him to place some tulips on his and his wife's graves whenever Naruto thinks of them.