Research from Barclaycard suggested that one third of all online sales were made between 6pm and 9am.
The rise in twilight shopping is part of the increasing popularity of online or e-shopping, estimated to be growing by 90% a year in the UK.
The increase contrasts sharply with the plight of more traditional High Street shopping, which economists warned this week was showing weak growth.
Shopping by moonlight
Barclaycard said its report revealed the changing trends in UK shopping, as many people found they had less time to step out on the High Street.
"People with busy lives often don't have a lunch hour to go shopping or might want to make a big purchase, like a holiday, when they are at home with their partner," the report said.
The report suggested travel and financial service products were the most popular online purchases with night owls, and that the average spend was £1.60 higher than during the day.
"The internet has significantly affected our shopping habits," it said.
The report estimated that the volume of online shopping in the UK would hit £7bn this year, or 4% of all UK retail spend.
No cash?
The boom in e-commerce defies figures this week from the Confederation of British Industry (CBI) which found a weakening in High Street sales.
Alastair Eperon, chairman of the CBI's survey panel, said: "Consumers are either being careful with their money or have less to spend".
When it comes to shopping online, however, it seems shoppers can be persuaded to spend.
Internet research group Jupiter MMXI told BBC News Online that websites were winning more business through "impulse purchases, especially at the weekend".
The group added: "Convenience is the main driver, but the consumer often browses for fun and ends up buying something."
Making it safe
Jupiter estimates that online sales in Europe would be worth $28bn in 2003, against $20bn this year.
The main hurdle to an even greater explosion in online sales was the lingering security fear, with many shoppers reluctant to reveal their credit card details online.
But an EU directive which came into effect in August set strict security measures for UK businesses advertising or selling online.
The Department of Trade and Industry said the trend was improving perceptions of the internet as a safe medium through which the shop.
A DTI spokeswoman told BBC News Online :"The internet is safe as far as we're concerned, as long as you follow the guidelines."
E-shopping, according to the DTI , is "quick, convenient" and offered "potential bargains".