A new study suggests that a sleep disorder known as sleep drunkenness is more common than previously assumed and affects one in every seven people.

People suffering from sleep drunkenness wake up confused. They might answer the phone or the doorbell instead of an alarm. For a long time, these behaviours got less attention and were considered as a rare condition. Now, researchers at the Stanford University School of Medicine have found that several people experience confusion after waking up from deep sleep.

The study included 19,136 people aged 18 years or older, living in the U.S. Participants were asked about their sleeping habits, mental disorder and medication use.

The researchers found that 15 percent of the study group had suffered an episode of sleep drunkenness in the last year. At least half of these people even reported having more than one episode a week. Around 84 percent of the people also had sleep disorders, a mental health disorder or were taking psychotropic drugs. Less than one percent of sleep drunkenness patients didn't have a related sleep or mental disorder.

About 37.4 percent of the people who reported experiencing at least one episode of confused arousals had a mental disorder. According to the researchers, patients suffering from depression, bipolar disorder, alcoholism, panic or post-traumatic stress disorder are more likely to experience sleep drunkenness than other people.

"These episodes of confused awakening have not gotten much attention, but given that they occur at a high rate in the general population, more research should be done on when they occur and whether they can be treated," said Maurice M. Ohayon, MD, , with Stanford University School of Medicine, lead author of the study, according to a news release. "People with sleep disorders or mental health issues should also be aware that they may be at greater risk of these episodes."

The study is published in the journal Neurology.