Nearly one in three overnight workers are aged over 50, according to analysis by one of the UK's largest unions.
Over-50s account for about 924,000 of a record 3.2 million people who regularly work through the night, the Trades Union Congress (TUC) says.
It wants better protection for night workers' health and wellbeing, saying the government does not do enough.
The government said it was proposing new rights for flexible workers, including those who work night shifts.
Jobs most likely to involve night work include care work, nursing, road transport and security, the TUC said.
The union has called for greater protection for those workers, saying that the hours can have a damaging effect on family life, as well as physical and mental health.
According to its analysis, the number of people regularly working night shifts is at its highest level since current official records began in 2005.
It found that about 100,000 more people are regularly working overnight than five years ago.
It found that 173,000 more over-50s were working overnights than in 2014.
The research also indicated that there are 222,000 people over the age of 60 working nights, and 69,000 over 65.
The union said its findings were taken from analysis of information collected in the Office for National Statistics labour force survey.
The survey of 38,000 households in the UK provides information on the UK labour market.
General secretary Frances O'Grady said: "Britain's loyal army of night workers has been boosted significantly by older workers.
"We all owe them a huge debt for keeping the country ticking over while we are asleep.
"Night work can be really hard - disrupting family life and placing a strain on people's health."
A government spokesperson said it was "determined to make the UK the best place in the world to work and start a business" and that older workers "provide a huge benefit to our economy".
They said the government was proposing new reforms that would see all flexible workers, including night workers, benefit from new rights and protections.
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