 "Sunday Morning" was written by Kanal, Gwen Stefani, and Eric Stefani, while production was handled by Matthew Wilder. The song is a ska punk and reggae rock song, with one critic comparing it to Motown music. According to Chris Heath of Rolling Stone, the single "erupts delightfully from its opening harmonies into a thumping combination of Motown and pop cheese, is especially spirited"; he later compared it to Kim Wilde's 1981 single "Kids in America". Kenneth Partridge, writing for Billboard, stated that the song allowed Gwen to go "from victim to victor", especially in the lyrics "Thank you, now you're the parasite"; in Partridge's The A.V. Club review, he described it as "a straight-up ska track" that "didn't chart as high, but the damage was done".
