"What do you believe in?" "Sex and death. Two things that come once in a lifetime. But at least after death, you're not nauseous."

The BFI Woody Allen retrospective continues tonight with Sleeper, one of Allen's earliest films - and one of the most quintessential. From its opening scene, in which a man (Woody Allen) is woken up in the future after being cryogenically frozen (complete with signature glasses), you can tell it's going to be a very silly affair. You'll also note a lot of the elements that have popped up in Austin Powers, as well as in Allen's future films.

Awake and disoriented, Allen's health food store owner staggers around the 22nd Century lab, eating rubber gloves and running people over in a wheelchair. All the while, he wears a manic grin on his face. It's genuinely hilarious, clearly inspired by comedians such as Benny Hill and Buster Keaton, as loud jazz honky tonks over the top of near-silent slapstick. Early on, instant pudding takes over the kitchen; later, Allen flies away from government officials before going on to win the Miss America beauty pageant. 

Sleeper is stitched together loosely, a patchwork of gags and occasional plot. Satire is in there somewhere too, but so are giant bananas, scientific experiments and machines that cause orgasms. It's the epitome of Allen's "early, funny ones" and he has rarely exhibited such an open love of anarchy since.

In that sense it compares well to the initial output of fellow silly-to-serious helmer Pedro Almodovar.

Like Labyrinth of Passion, culture and sex collide in Sleeper's chaotic 80 minutes, establishing themes and styles of the filmmaker's later work; Allen's Bullets Over Broadway, for example, shows a continued love of farcical chase sequences and Love and Death rivals Woody's wordless seduction of women. More recently, Midnight in Paris takes an equally satirical look at the past with absurd, fast-paced verbal humour.

"It's hard to believe that you haven't had sex for 200 years," says Diane Keaton back in 1973. "204, if you count my marriage," comes the inspired reply. That kind of wit is still there today - but it's often swamped by serious melodrama. In Spain, Almodovar learned to balance these two tones to produce masterpieces in All About My Mother and Volver. Allen, meanwhile, has seen spurts of brilliance in between Match Point and Scoop.

As well as establishing Allen's comic vein, Sleeper sparks another big trend for the director by introducing Diane Keaton to his work. Their chemistry jumps off the screen, his awkward presence and her flawless timing making their partnership believable. That effortless unison would produce classics such as Annie Hall, all the way up to A Midsummer Night's Sex Comedy, when Mia Farrow entered the frame.

Sleeper also showcases Allen's knack for naming a project. One-word titles litter his back catalogue, but whether it's short like Bananas or long like Midsummer Night, his titles always seem to capture something of the movie's style. Sleeper succinctly sums up its sci-fi premise while allowing room for intrigue, while Crimes and Misdemeanors echoes the seriousness of Dostoevsky and The Purple Rose of Cairo recalls the romance of matinee movies. You Will Meet a Tall Dark Stranger, on the other hand, suggests something shallow, false and clunky.

Of course, like Pedro's Labyrinth of Passion, Sleeper is not as mature as the director's later work - Hannah and Her Sisters' intelligent mix of humour and emotion is perhaps the equivalent of the Spaniard's Women on the Verge of a Nervous Breakdown. You could also complain that it lacks an ending, but like Monty Python and the Holy Grail, Sleeper's string of sketches simply do not date, remaining some of the funniest to ever hit the big screen. That's a pretty good trend for any director to establish. 

For more on Woody Allen at the BFI and to book tickets for Sleeper tonight, visit the season's official site. Alternatively, read our reviews of Hannah and Her Sisters and Zelig, both now on re-release in the UK by Park Circus on lovely 35mm prints.