Parks in the city of eight million people are now plunged into total darkness after 11 pm, while two-thirds of street lights are also switched off at the same hour.
Scientists have warned that global warming is intensifying extreme weather events such as heatwaves.
In early May, Vietnam recorded its highest-ever temperature -- 44.1 degrees Celsius (111.38 degrees Fahrenheit) -- breaking a previous record set in 2019.
The country sweltered under a heatwave in April, and another in late May, and state electricity company EVN has warned that huge demand from air conditioners and fans has put the national power system under strain.
Adding to the problems, a severe drought in northern Vietnam means water levels at hydropower dams are 30 to 40 percent lower than normal.
"I am worried about a power shortage, which may badly impact us during the hot summer," Hanoi resident Do Tung Duong said while on a walk in the dim city centre.
Another resident, Vu Thi Hoa, told AFP she agreed with the measure to cut public lighting.
"We should turn off unnecessary electric equipment, especially the lights. It feels hotter if there are too many lights on," she said.
"We need power for fans and air conditioners. It will be terrible if there is a power cut."
Hanoi Public Lighting Company HAPULICO reduced the city's street lights in response to EVN's calls for energy saving.
Public lighting is switched on half an hour later than usual, and turned off half an hour sooner.
Although some cuts are made to street lights every year, "the power saving scheme is in a wider area this year, covering 70 percent of the city's public lighting system," HAPULICO deputy director Le Trung Kien told local media.
"We still ensure enough lighting for traffic, security and order."
HAPULICO said the cuts may last until the end of August.
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