Venusian lullaby

You may be looking for the novel.

A Venusian lullaby was used by the Third Doctor to soothe Aggedor, along with the spinning mirror/torch, which apparently had a hypnotic effect on the beast. (TV: The Curse of Peladon, The Monster of Peladon)

For reasons unknown, the melody of the lullaby was similar to a centuries-old Earth Christmas carol entitled, "God Rest Ye Merry, Gentlemen". As the original composer of the carol -- and indeed the date of its creation -- have been lost to history, any connection that may exist between the two pieces of music can only be speculated on.

The first line translates roughly to "Close your eyes, my darling. Well three of them, at least"! (TV: The Dæmons)

More obscurely, in the late 2000s, an illustrated book of Venusian lullabies was used by Rhys Williams to placate a giant Zansi baby when Gwen and he took care of him. (COMIC: Rift War! Part Three: Funhouse!)

The Eleventh Doctor used the lullaby when he organised an orchestra to prevent the Drexxons escaping from their cell in a Perpetuity Chamber, which was locked by a musical key. (PROSE: Death Riders)

Some archaeological sources suggested that the tune wasn't a lullaby, but instead a bawdy rhyme. (PROSE: Lucifer Rising)

Behind the scenes

• Jon Pertwee (the Third Doctor) liked to say he has made up the tune to the song by putting nonsense words to "God Rest Ye Merry, Gentlemen". If this is true, and the song wasn't devised by an episode writer or house composer, this would give Pertwee the distinction of having written a song for Doctor Who (or at least the lyrics to one).
• The original Christmas carol has since been heard several times in the 2005-present revived series.