Long hours on the job means a higher likelihood of high blood pressure, recent research shows. But it can be doubly dangerous: It also increases the chances of masked hypertension, a form of high blood pressure that doesn’t appear during a medical visit.

A recent study by Canadian researchers show the acute effects that grueling work hours can have on blood pressure.

They tracked a group of more than 3,500 white-collar workers, monitoring their blood pressure at various points during a five-year period. Those who worked at least 49 hours a week had a 70% greater chance of masked hypertension and a 66% greater likelihood of sustained high blood pressure compared with workers who put in 35 hours a week or less.

Workers who logged 41 to 48 hours a week also showed significantly higher risks of both types of hypertension.

Masked hypertension is a condition in which blood pressure readings are normal during health checkups but elevated at other times such as during working hours. Both masked and sustained high blood pressure are linked to higher cardiovascular disease risk.

To simulate in-clinic blood pressure measurements, study participants wore monitors to record their blood pressure in the mornings and then again at various points throughout the work day. Overall, nearly 19% of the participants had sustained hypertension, and more than 13% had masked hypertension.

For those who work long hours, researchers said there’s a simple takeaway message: Ask your doctor about a wearable monitor to record blood pressure over time. Masked hypertension can be a hidden hazard, with about 20% of people never showing high blood pressure during a medical checkup.