Overtime work, insufficient sleep, and risk of non-fatal acute myocardial infarction in Japanese men

Objectives: To examine the relation between working hours and hours of sleep and the risk of acute myocardial infarction (AMI), with special reference to the joint effect of these two factors. Method: Case-control study in Japan. Cases were 260 men aged 40-79 admitted to hospitals with AMI during 1996-8. Controls were 445 men free from AMI matched for age and residence who were recruited from the resident registers. Odds ratios of AMI relative to mean weekly working hours and daily hours of sleep in the past year or in the recent past were calculated. Results: Weekly working hours were related to progressively increased odds ratios of AMI in the past year as well as in the past month, with a twofold increased risk for overtime work (weekly working hours greater than or equal to61) compared with working hours less than or equal to40. Short time sleep (daily hours of sleep less than or equal to5) and frequent lack of sleep (2 or more days/week with <5 hours of sleep) were also associated with a two to threefold increased risk. Frequent lack of sleep and few days off in the recent past showed greater odds ratios than those in the past year. Conclusions: Overtime work and insufficient sleep may be related to increased risk of AMI.