"Sunnyday" was used as the theme song to the second season of the anime series Blue Dragon.

[1] It was also their only single released for their compilation album Lead Tracks: Listener's Choice, which was an album composed of songs chosen by fans throughout East Asia.

[2] The single was only released as a standard CD, which held "Sunnyday", the edited version of the track and an instrumental.

[4] While Seiji composed the piece, the members of Lead - Hiroki, Shinya, Keita and Akira - wrote the lyrics, with Shinya writing the rap portion.

[5] On February 27, 2012, Lead's official YouTube uploaded the music video.