Human activity heats up cities to such an extent that on the weekends when we're having a sleep-in, the centre of town is measurably cooler.

It's a different story at 9am on weekdays when work draws people into the CBD, according to research from Melbourne and Monash universities.

Researcher Nick Earl in Melbourne at the coolest time of the week - early Sunday morning. Credit:Luis Ascui

"Everyone has a lie-in on Sunday and that is reflected in a city's temperature," said Nick Earl from Melbourne University's school of earth sciences.

The findings revealed that 9am on a Sunday morning was the "coolest" time to be in town, with the city's air temperature up to 0.3 degrees milder than an average Thursday or Friday morning at 9am – which were identified as the warmest days of the week.