Northlanders tossing and turning in bed will have a brief respite from the humidity for a few days before sleeping becomes uncomfortable again.
Night-time temperatures at many places across the nation this month - particularly in the upper North Island - have been well above normal, says Niwa.
Temperatures in places such as Dargaville have been above or well above normal and just below normal in areas like Warkworth.
Those planning a day out during the two upcoming public holidays - Northland anniversary on Monday and Waitangi Day on February 6 - will have plenty to enjoy.
Niwa forecaster Chris Brandolino said most of Northland would experience near or above temperatures over the next two weeks, with a typical maximum of 24C and minimum of 15C.
He said those near water would feel more humidity because the ocean temperature, particularly in tropical countries, was usually in the lower 20s.
"Starting yesterday, humidity levels in Northland should come down and, by Wednesday next week, there's a good chance those levels should start climbing back up again," he said.
"So people should feel more comfortable sleeping in the next few days in a drier atmosphere. The temperatures will come down a fraction in the new few days but the humidity levels would fall substantially."
Mr Brandolino said pockets of cooler and drier air that were moving north and east along the eastern half of the country would reach Northland, resulting in lower humidity levels.
People heading to bed at 9pm would still be finding the air outside warmer than they might like but the temperature tended to drop sharply between 11pm and midnight.
The "day after" effects of poor sleep included problems with concentrating, motivation, co-ordination, or getting on with other people.
Temperatures in Northland have been above average this summer and the MetService has predicted the high humidity to stick around for at least the rest of this month.
A slow-moving weak front over Northland is bringing warm and moist air and isolated showers.