Facing a power crunch and a scorching summer, some of nearly 10,000 Tokyo city government employees started their work shifts an hour earlier on Monday to conserve energy amid shortages caused by the damage to a tsunami-hit nuclear plant.

Tokyo government workers on the earliest shift start at 7:30am and 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 on Friday. "I think all of Japan should shift to the summer time hours."

To prevent blackouts in the wake of the 11 March disaster, which knocked out Tokyo Electric Power Co.'s Fukushima nuclear power plant, Japan's central government has asked companies and government offices to cut electricity usage by 15%. It wants companies to limit air conditioning and set room temperatures at a warm 28C (82F).

Officials are also encouraged to follow a new dress code called "Super Cool Biz" launched last week that urged employees to wear lighter clothing, such as polo shirts, Aloha shirts and even sneakers 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.

Tepco expects to supply 53.8m kilowatts (kW) for Tokyo and its vicinity in July, which is short of an estimated demand of 60mkW. Tokyo uses one-third of Tepco's output.

Ishihara has set a more aggressive target of 25% reduction in energy use for Tokyo, and officials are hoping that government workers' new hours will spill over to the private sector.

"To set a good example, we shouldn't keep lights on at our offices until late," said city official Hideo Ishii.

Some 9,500 employees at the city's headquarters will be fully participating by the end of the week. By July, that number will grow to 25,000. It will exclude teachers, police, firefighters and medical experts.

While employees will start work earlier, the clocks inside the city hall won't change.

Still, Ishihara would like to take the experiment a step further and establish a yearly daylight saving time, shifting clocks forward during summer as many Western nations do.

Changing the clocks in the summer is debated in Japan, with most sentiment opposed.

Daylight saving time was imposed in 1948 under the US occupation after Japan's defeat in world war two, but lasted only four years because it was so unpopular and has since been for years remembered negatively.

More importantly, corporate culture in Japan is such that many workers feel obligated to work until it is dark outside – no matter what their starting time.