“It’s about learning to be comfortable being uncomfortable,” said England boss Roy Hodgson
Experts from Loughborough University are helping monitor players during training in Portugal.
“We tested how much they sweat, how they will be able to recover and what can be done to help,” added Hodgson.
England face Italy in their World Cup Group D opener in the jungle city of Manaus on 14 June, before taking on Uruguay and Costa Rica.
“We started off on the first day just with them wearing the extra gear for the warm-ups,” said Hodgson at England’s training camp at Vale Do Lobo. “Then we’ve increased it.”
Temperatures can top 30C in Manaus in June but humidity levels can reach more than 80%.
Temperatures at the England training base on Wednesday were about 15C.
Dave Reddin, Hodgson’s head of performance services, recommended the players wear special pads underneath their tracksuit tops so England’s medical staff and three sports scientists from Loughborough University could assess how the squad responded to sweating large amounts.