Heatstroke was exacting a toll as the nation continued to swelter Wednesday.

The mercury topped 40 in Yamanashi Prefecture and remained above 30 through the previous night in Tokyo for the first time since 1951, when the Meteorological Agency started keeping track, agency officials said.

In Tokyo, 48 people were hospitalized with heatstroke on Tuesday, the Tokyo Fire Department said Wednesday.

Three people were in serious condition, including a 17-year-old high school student who was playing soccer in Nerima Ward when she became sick, according to the department.

The remaining 45 suffered from milder symptoms.

Hospitals have treated 392 people between July 1 and Tuesday due to the sizzling temperatures.

The Tokyo overnight low of 30.1 was marked around 4:30 a.m. Wednesday, exceeding the previous record of 29.3 on Aug. 4, 1994, according to the agency.

Nationwide, it matches the record high overnight low marked in Toyama on July 31, 2000, according to the agency.

Various other parts of Japan also continued to experience sweltering temperatures through the night.

In Kofu, Yamanashi Prefecture, the mercury rose to 40.1 shortly after 1 p.m., passing Tuesday's high of 39.9.

"The so-called heat island phenomenon is likely one of the reasons why temperatures in Tokyo alone had not markedly decreased," an agency official said, referring to the heat urban areas give off compared with rural areas, mainly due to concentrated development and massive energy consumption.