Simplify the following text for a Grade 7 native English-speaking person.
Original: A study by Knutsson et al in 1986 showed that people who work shift work for more than 15 years , are 3 times more likely to develop ischaemic heart diseases .
Simplified: A study by Knutsson et al . in 1986 found that shift workers who had worked in that method for 15 years or more were 300 % more likely to develop ischemic heart disease .