The fish market opens for 24 hours a day, but it is usually most crowded from midnight to 6 a.m.
Every day more than 100 tonnes of fish are delivered throughout the capital and to several neighbouring areas from this market.
The temperature falls to nine degrees Celsius at 1 a.m. on January 25.
A trader makes a living at Yen So market.
Many traders don raincoats as they struggle to cope with the chilly winter weather conditions which have hit the capital.
The site is the largest fish market in Hanoi, with nearly 100 stalls in the market.
Whilst the whole city is sleeping the fish market is at its busiest time, even in rain and cold weather.
Fish sold at the market comes from the northern provinces of Hung Yen, Hai Duong, and Ha Nam.
Workers are busy at night at Yen So market.
Traders burn a fire as a means of keeping warm at Yen So market.