In the 1920s, poet Federico Garcia Lorca, who studied Spanish lullabies, noted the "poetic character" and " depth of sadness" of many of them.

BBC: The universal language of lullabies

Lorca's theory - which many researchers today would agree with - was that a large part of the function of the lullaby is to help a mother vocalise her worries and concerns.

BBC: The universal language of lullabies