Many Tokyo local government staff are starting work an hour early in an attempt to save power because of the shortages caused by the country's damaged nuclear network.
They were in their offices at 7.30am allowing them to leave at 4.15pm.
By better exploiting the early daylight hours this summer, city officials hope to use less air conditioning and office lighting at night.
"It should be a good thing, and it doesn't require any cost," Tokyo's governor Shintaro Ishihara said. "I think all of Japan should shift to the summer time hours."
To prevent blackouts in the wake of the March 11 disaster, which knocked out the Fukushima Dai-ichi nuclear power plant, Japan's central government has asked companies and government offices to cut electricity use by 15%.
It wants companies to limit air conditioning and set room temperatures at a warm 28C.
Officials are also encouraged to follow a new dress code called "Super Cool Biz" that urges employees to wear lighter clothing instead of the traditional tie and jacket.
Households across Tokyo are advised to use electric fans instead of air conditioners, unplug appliances when not in use, and raise temperature settings on refrigerators.