People living in Splott say they are being kept awake by work to electrify the South Wales main rail line.
Network Rail have written to residents to apologise for the disruption caused by loud equipment and bright floodlights.
They say working at night is their only option.
Kim Nieuwenhuis, who lives near the line, said she could not believe how loud the works are.
Ms Nieuwenhuis said: “I understand that they have to get the work done but it’s really noisy at 3am so I don’t really think that’s good enough.”
The work in Splott is part of an upgrade by Network Rail which prepares the line for overhead wires and posts to power trains running on electricity between south Wales and London.
But one resident, posting on Twitter at 5.30am on a Sunday morning said it “sounds like someone playing drums on a metal bucket.”
