German scientists have found the combination of in-flight alcohol and cabin pressure at cruising altitude may put strain on sleeping passengers' hearts, Sky News reports.

They discovered that when people fall asleep after drinking alcohol at the low air pressures typically experienced during flights, blood oxygen levels drop and heart rates increase - even in those who are healthy and young.

Writing in the journal Thorax, the researchers said passengers with heart problems have an increased risk of aggravation of symptoms due to the decreased cabin pressure at cruising altitude, which is amplified during sleep.

Alcohol, often consumed on board, has similar effects, they said.

"The on board consumption of alcohol is an underestimated health risk that could be easily avoided," academics from the Institute of Aerospace Medicine in Germany said. "It may be beneficial to consider altering regulations to restrict the access to alcoholic beverages on board."

For the study, researchers tested the impact of alcohol consumption and sleep in a hypobaric environment—one with low air pressure.

They assessed 48 people aged between 18 and 40 over two nights in two different environments—a sleep laboratory and an altitude chamber.

Participants consumed alcohol before one of the nights.

Researchers found the combination of alcohol and experiencing low oxygen concentration at high altitudes reduced sleep quality, "challenged the cardiovascular system" and led to extended duration of low blood oxygen levels.

The authors concluded: "Together these results indicate that, even in young and healthy individuals, the combination of alcohol intake with sleeping under hypobaric conditions poses a considerable strain on the cardiac system and might lead to exacerbation of symptoms in patients with cardiac or pulmonary diseases."