Doctors are less likely to prescribe pain medication at the end of a long night shift than they are at the start, a new study reveals – perhaps pointing to a lower level of empathy for patient pain when physicians are themselves feeling worn out.

Researchers behind the study are calling for hospitals to do more to manage the schedule and the workload of health professionals, so that clinical decisions aren't affected by fatigue in these kinds of ways.

Even just making physicians aware of potential bias at the end of their shift could be helpful, the researchers suggest. More robust guidelines for the prescription of pain medication might also be useful in ensuring patients get the treatment they need.

"Our takeaway is that night shift work is an important and previously unrecognized source of bias in pain management, likely stemming from impaired perception of pain," says psychologist Anat Perry from the Hebrew University of Jerusalem in Israel.

"Even medical experts, who strive to provide the best care for their patients, are susceptible to the effects of a night shift."

The study involved 31 resident physicians in Israel who were just starting work, and 36 who were about to finish a 26-hour shift. These participants were given invented scenarios detailing a female patient with a headache and a male patient with backache.

Those doctors coming off their shift rated the pain of the patients as less intense, and were less likely to prescribe analgesics to relieve pain, in these hypothetical scenarios. It would seem that fatigue plays a part in making a call on how much someone else is suffering, and therefore the kind of treatment they need.

The study also included an analysis of 13,482 discharge letters for patients in the US and Israel, collected over a period of seven years. Across all the data sets, physicians were 20–30 percent less likely to prescribe pain medication during a night shift.

"The fact that the divergence of analgesic prescription from the general World Health Organization guidelines is greater during night shifts suggests that there is indeed an under-prescription during night shifts, rather than an over-prescription during daytime," says pediatric pulmonologist David Gozal from the University of Missouri School of Medicine.

The bias was still present after controlling for the level of pain reported, the demographics of patients and doctors, the type of complaint, and variations in emergency departments, the researchers behind the study say.

With pain being one of the main reasons people see a doctor – and almost 60 percent of US adults experiencing pain within the last three months – it's important to make sure it's properly managed and prevented from developing wherever possible.

Various biases (including race and gender) can affect the prescription of analgesics, and now night shifts can be added to that list. What's more, it's the latest in a long line of studies demonstrating how harmful night shifts can be if they're not managed properly.

"These results highlight the need to address this bias by developing and implementing more structured pain management guidelines and by educating physicians about this bias," says Gozal.

The research has been published in PNAS.