We’ve all seen it. You’re at a major airport preparing to catch a 7 a.m. flight, you pass a bar and it’s full of people drinking. Or you settle into your seat and your neighbor is desperately flagging a flight attendant for a pre-takeoff gin and tonic.

Whether it’s trying to calm frazzled-flyer nerves, kick-start a vacation, or just hoping for sleep, many air passengers opt for cocktails or a glass of wine to pass the time.

However, a study from the U.K. says you put yourself at risk when drinking alcohol on airplanes, especially on longer flights when you plan to sleep.

That’s because alcohol and altitude are a bad mix. As the airplane climbs higher, the cabin’s oxygen level drops. This causes your blood oxygen level to decrease.

And it makes your heart work harder. Researchers found study participants had an elevated heart rate after drinking and falling asleep, compared with their sober-flyer friends. Both lower blood oxygen levels and increased heart rates are telltale signs of cardiovascular strain.

Some passengers use alcohol as a sleep aid. While imbibing may help you fall asleep faster, your quality of sleep deteriorates as your body works to breaks down alcohol. You wake up more often and you’ll likely feel worse when you do.

So, what’s a tired traveler to do? Experts say it’s best to avoid alcohol before and during a flight. That is especially so if you have a heart or lung condition.

Fortunately, there are other ways to relax. Consider a travel pillow, an eye mask, and ear plugs. Headphones with calming music or white noise can help. Just don’t go looking for rest in the bottom of a glass.