Just over 20 years ago (July 29, 1991), 'Enter Sandman' was the first single released from Metallica's self-titled 'Black' album. In a recent interview, producer Bob Rock says the group labored over even the most particular details, especially the lyrics, in an effort to make the song perfect. Clearly their efforts paid off.

The record, the already supremely popular band's fifth, broke them through to an entirely new level of stardom, eventually becoming the best selling album in the Soundscan era, after spending almost the remainder of the decade on Billboard's Top 200 album chart.

Rock tells MusicRadar.com he retooled the lyrics with James Hetfield, who was writing a much darker song at first. “I sat down with James and talked to him about his words. I told him, ‘What you have is great, but it can be better. Does it have to be so literal?’" The band's management was hoping the song would be the album's first single but, "Then they heard James’ lyrics and realized the song was about crib death. That didn’t go over well."

The final lyrics are more about a child's fantasized death, a cryptic fairy tale, than actual tragedy. "Say your prayers little one / Don't forget my son / To include everyone / I tuck you in / Warm within / Keep you free from sin / 'Til the sandman he comes."

In a 2008 RollingStone.com interview, guitarist Kirk Hammett talked about writing the song's ever-lasting riff. "Soundgarden had just put out Louder Than Love. I was trying to capture their attitude toward big, heavy riffs. It was two o'clock in the morning. I put it on tape and didn't think about it. When (drummer Lars Ulrich) heard the riff, he said, 'That's really great. But repeat the first part four times.' It was that suggestion that made it even more hooky."

The tension built by this riff pays off with a heightened sense of release when the now-famous chorus hits: "Exit light / Enter night / Take my hand / We're off to never never-land."

“I insisted on the band playing live in the studio." Rock says. "They had never done that before – all of their previous records were recorded in sections. I told them, ‘You’re a great live band. That vibe is crucial to the album.’ It's rumored that Rock joins Hetfield in the child's prayer toward the end of the five-and-a-half-minute song. "Now I lay me down to sleep / Pray the lord my soul to keep / And if I die before I wake / Pray the lord my soul to take."

Every bandmember, Rock adds, was asked to stretch his talents, and play in a way previously unnatural to him. “On Sandman, I asked Jason to play more like a bass player and less like a guitarist. Put that with the new perspective Lars had on drums and we had a song with a killer groove."

Watch Metallica Perform 'Enter Sandman'