Making workers available on call can make sense for a number of reasons. It provides the employer with an increased labor capacity without having to pay full wages to idle workers. Employees, meanwhile, can get paid without even being at work.

But just because on-call workers are in the comfort of their own home doesn’t mean there’s no toll to waiting by the phone. Even if they don’t get called into work, they end up being more tired after an on-call shift because their sleep is affected by the fear they might not hear the call coming in. One study notes that the “mere possibility of being called heightens the need for recovery among shift workers.”

Consecutive sleep is required for the body to function normally. In the first three to four uninterrupted hours of sleep your body releases a growth hormone that repairs and helps the body recover from stress. Employees who work shift work are often sleep deprived as they try to combat their body’s internal clock. And research shows that being awake 17 to 19 consecutive hours is comparable to having a blood-alcohol level of .05 percent.

Even if workers aren’t awake for that long, any change in sleeping patterns can affect their safety and productivity at work because fatigue affects mental alertness, reaction time and physical reaction time.

You may not be able to eliminate the fatigue-inducing effect of being on call, but it’s possible to reduce the overall impact of fatigue on your workforce with human factors training.

This training can help employees identify when they’re in the state of fatigue and show them how to avoid making errors because they’re tired.