The Tokyo 2020 Olympic marathon will start between 5:30am and 6am, organisers announced Wednesday, after experts raised concerns over the health of competitors and fans during the city's sweltering summer heat.
An expert medical committee had proposed a start time "between 5:30am and 6am", top Olympic official John Coates said after a series of meetings in the Japanese capital.
Organisers had already brought the marathon forward to 7am as Japanese medical groups have warned the event could "lead to deaths" from heatstroke.
Tokyo 2020 organisers still need to agree with the International Association of Athletics Federations before fixing the exact time, said Coates, who is chairman of the Olympic coordination commission.
Rugby sevens matches will also start 90 minutes ahead of schedule and mountain biking an hour later to avoid the burning sun. The 50-kilometre race walk will also be brought forward.
The Japanese capital sweated through several deadly heatwaves this summer as Japan was also battered by a series of typhoons that caused death, destruction and delay.
Coates said a series of anti-heat measures was being planned, including mist sprays along the marathon route and heat-absorbing paint on the roads.
There is also an increase in medical facilities as a precaution and more shade for fans lining up for entry to the venues.
"There is a list of about 20 precautions they (the medical commission) think we should take and they are not going to be free," quipped Coates.
Indeed, Tokyo 2020 CEO Toshiro Muto admitted that anti-heat measures had increased pressure on the Tokyo 2020 budget, which is due to be updated on December 21.
Olympic organisers are under huge pressure to keep a lid on costs amid fears that spiralling budgets for the event are sapping public support.
"We can't exceed the level of the previous version of the budget," said Muto, who added that cuts had been found in other areas to compensate for additional spending against the heat.
The Olympics are expected to cost around $12.6 billion, according to the current version of the budget, unveiled last year.
The potential for extreme weather conditions has become the major headache for organisers of the event which opens on July 24.
The previous time Japan hosted the Olympics, in 1964, the competition were held in October to avoid the hot summer conditions.
Organisers have toyed with several ways of beating the heat -- even proposing the introduction of daylight saving time, an idea that appears to have fallen by the wayside.
Japan adopted daylight saving in 1948 under US occupation after defeat in World War II, but scrapped it four years later following concerns it was encouraging longer work hours.
Coates noted that this summer was abnormal, with the mercury touching a record 41 degrees Celsius (105.8 Fahrenheit) during the hottest periods.
The hottest day of Olympic competition is the 36 Celsius measured at the 2004 Athens Games.