Its research into the ONS Labour Force Survey suggested that the number had increased 200,000 since 2007 to just under 3.2 million people.
That is a rise of 6.9% over a period when the workforce grew by about 4.6%.
The report was released ahead of a strike by Tube workers who are unhappy about London Underground's plans to introduce all-night services.
The increase in the number of people saying it is normal for them to work at night does not necessarily mean there is more work being done at night, because no account is taken of how much night work is done by each respondent.
Men are still considerably more likely to work at night, with about 15% of male workers doing night work compared with about 10% of female workers.
The largest numbers of night workers were employed as care workers, nurses and midwives.
The TUC warned about the impact of working at night on both workers' health and their family life.
"We all value night workers, whether they are cleaning our office, caring for a sick relative or driving all night so that there are fresh goods in our local shop," said TUC general secretary Frances O'Grady.
"But night work is hard and it disrupts family life, so we must show our appreciation for the sacrifices night workers make by ensuring they have sensible rights and protections."
The Tube strike will begin at 1830 on Wednesday.
The number of UK workers who routinely work nights is on the rise, according to the TUC.