(continued excerpts from Sheffield's article)

[…]

Given the album’s impact […], it’s startling to note how fast and frantic the sessions were. The Beatles didn’t go into the studio with a mystic crystal vision to express — they went in with a deadline. They had to supply product for the 1965 Christmas season, which meant crunching it out in four frenzied weeks, from October 12th to November 12th. So they holed up in Abbey Road around the clock, pouring out music as fast as they could, holding nothing back. They were willing to try any idea, whether it turned out brilliantly (the sitar, the harmonium) or not (the six-minute R&B instrumental jam, which they wisely axed). They wrote seven of the songs in one week.

But with their backs against the wall, working under this pressure, the Beatles produced an album that was way ahead of what anyone had done before. Since these guys were riding new levels of musical fluidity and inspiration, firing on so many more cylinders than anybody else had, they stumbled onto discoveries that changed the way music has been made ever since. It was an accidental masterpiece — but one that stunned the Beatles into realizing how far they could go. After that, they went full-time into the masterpiece-making business.

[…]