Proust ran his life on a clock that turned backwards. He slept while Paris worked and worked while Paris slept, and he kept that inverted schedule going for years so he could write, in bed, through the deep quiet of the night. It sounds like affectation. It was closer to survival.

The Direct Answer
He lived by night. Proust slept through the day and wrote in bed until dawn.
In his later years he'd wake in the late afternoon and work through the small hours, a routine detailed by his housekeeper Céleste Albaret in Monsieur Proust.

The Day Turned Upside Down

The reversed schedule began, like almost everything with Proust, in his lungs. His asthma tended to be worse in daylight, when the air carried more pollen and dust, so he pushed the hard work of living into the dark. Inside his cork-lined, curtain-black bedroom there was no daylight to argue with anyway. He would often go to bed around seven in the morning, just as Paris was waking, and surface again in the late afternoon.

It's easy to file this under eccentric-genius trivia, but there was a hard logic under it. Day meant symptoms and visitors and noise. Night gave him air he could breathe and a house that finally held still. Proust wasn't drifting through his hours by accident. He engineered them, down to the clock, around the single job of getting the book written.

When he woke, Céleste Albaret, his housekeeper and confidante, brought what he called his breakfast: two large bowls of strong café au lait and two croissants, summoned by two rings of a bell. That was often the only real food he ate all day. He was perpetually cold and worked bundled in sweaters and hot water bottles, even in summer.

The reversal wasn't a stunt he could switch off. Proust had wrestled with insomnia since childhood, and by his later years his body clock had flipped completely. Céleste stayed up through the night in case he rang, and when he surfaced he might talk to her for an hour before settling to the page, replaying the evening or turning over what he meant to write next. She learned never to enter unless he called, and never to make a sound he hadn't asked for.

His social life ran on the same inverted clock. Proust would sometimes arrive at a dinner or a salon near midnight, staying only long enough to watch and listen, then head home to write while the impressions were still fresh. The late nights out and the late nights in were really the same job wearing two hats.

Writing Through the Night

The most famous sentence Proust ever wrote is about going to bed, which is either a coincidence or the whole man in a single line.

For a long time I used to go to bed early.

Marcel Proust, Swann's Way (trans. C. K. Scott Moncrieff)

Proust himself did the reverse. He wrote at night because that was when the building went silent and his body handed him a few usable hours. He worked propped up in bed, half sitting and half lying, notebooks balanced on his knees and the lamp turned low. The manuscript grew in that position, page by page, over more than ten years. For everything around the schedule, see the full Marcel Proust writing routine.

He held the pattern most tightly at the end. In the last three years of his life he was almost never out of that bedroom, pouring what he had left into the novel, correcting and expanding it night after night while his health gave way around him.

What You Can Steal