The Tokyo 2020 Olympic marathons will start at 06:00 local time to avoid extreme heat.
Organisers confirmed the schedule following warnings from Japanese medics that running in mid-morning could “lead to deaths” from heatstroke.
The possibility of extreme weather, including typhoons, is a “major issue”, said Tokyo 2020 chief executive officer Toshiro Muto.
The Olympics run from 24 July to 9 August.
The men’s 50km race will begin at 05:30, while the schedules for triathlon, rugby sevens and mountain biking have also been moved to avoid the highest temperatures.
When Japan last hosted the Olympics in 1964 they were held in October to avoid the summer heat.
The marathons began at 09:30 local time at Rio 2016, when the temperature was 19C.
Organisers have described the middle weekend of Tokyo 2020 – on 1 and 2 August – as “Super Saturday” and “Golden Sunday”.
