By James Reynolds
BBC News, Beijing
At an icy service station, close to midnight, you get a good idea of what goes on in Beijing when everyone else is at home in bed.
Xiao Hu only stops working when his truck breaks down
A row of trucks is parked by the side of the road.
On a clear night, they would be taking food, chemicals and supplies into the city centre.
But tonight, the fog is so thick you cannot see more than 10 metres in front of you. Until it lifts, the truck drivers simply have to sit and wait.
Xiao Hu stays in his cab and eats his way through a pot of steaming instant noodles.
His back-up driver sits beside him, drinking from a miniature bottle of spirits, pursing his lips after every sip.
Xiao Hu comes from the countryside. He bought his truck second-hand three years ago, and says he cannot afford to stop working.
"We work every night of the month," he said. "We only ever stop when the truck breaks down."
"As far as the future goes, I have to continue with this job. I have no other options."
At night in the centre of town, floodlights illuminate a building site. Workers in yellow hard hats carry steel rods towards the foundations of an office block, one of dozens going up in Beijing.
Millions of migrant workers have been drawn to China's cities
The construction workers come from villages in southern China. They work in shifts and sleep in cabins set up on the building site. Work goes on throughout the night.
Xiao Xu has been here for six months. He won't get paid till the building is finished.
"When it's cold and dark like this we want to press on," says Xiao Xu, "We want to get the building done so that we can go back to our villages."
After one in the morning, a handful of vendors sets up stalls outside the Lotus Market, next to a street of restaurants.
One woman shines a torch onto the packets of cheap cigarettes she is trying to sell. Two others try to convince passing tourists to buy the services of a masseuse.
Another woman, who does not give her name, sells toy dinosaurs that dance to a tune when you wind them up. She used to be a farmer but could not make a living in the countryside. So she came to Beijing and works overnight to try to avoid the police.
"They often come over to arrest us, and to confiscate our goods," she said. "So we can only do business very late, when they're not here."
A few streets away there is the sound of more construction work. It's past two in the morning. And the floodlights are still on.