Night owls might think staying up late is a real hoot, but a new study hints that delayed sleep might have a sinister side. People who hit the sack late might have a greater risk of experiencing nightmares, according to scientists, although they add that follow-up research is needed to confirm the link.

"It's a very interesting preliminary study, and we desperately need more research in this area," says Jessica Payne, director of the Sleep, Stress and Memory Lab at the University of Notre Dame, commenting on the new findings.

Previous reports have estimated 80 percent of adults experience at least one nightmare a year, with 5 percent suffering from disturbing dreams more than once a month. The new paper, from a group of scientists writing in the journal Sleep and Biological Rhythms, surveyed 264 university students about their sleep habits and frequency of nightmares, defined as "dysphoric dreams associated with feelings of threat, anxiety, fear or terror."

The scientists, led by Yavuz Selvi at the Yuzuncu Yil University in Van, Turkey, used a measure known as the Van Dream Anxiety Scale to assess the rate of bad dreams. Specifically, study participants were asked to rate their frequency of experiencing nightmares on a scale from zero to 4, corresponding to never and always, respectively.

On average, individuals who described themselves as evening types had a score of 2.10, whereas their morning-type equivalents averaged 1.23 on the scale, a significant difference according to the authors of the study.

The Turkish study follows from a larger online survey of nearly 4,000 people that found hints of an association between being a night owl and nightmares among women beginning in their 20s. Those results from Tore Nielsen, director of the Dream and Nightmare Laboratory at the Sacre-Coeur Hospital in Montreal, were published in 2010 in the Journal of Biological Rhythms. "I was pleased to see that they replicated the association between being an evening person and having nightmares," Nielsen says. But he adds that more research is needed into how this might play out according to a person's sex: "Men and women have very different emotional systems, and I think we're seeing a different expression of that difference in nightmares."

Experts are intrigued by the idea that a person's daily body cycle, known as the circadian rhythm, might be linked to nightmares. "I think it's certainly of interest," says Russell Rosenberg, director of the Atlanta School of Sleep Medicine & Technology and chairman of the National Sleep Foundation. "There's certainly not a lot of research in this area."

Mood matters

Exactly why evening types might report more nightmares is a mystery. The authors of the new study point to previous work that found these individuals might be more likely to have mood disorders and stressful lifestyles. Other scientists have found an association between mood disorders, such as depression, and sleep problems.

Mark Blagrove, director of the Sleep Lab at Swansea University, notes that the survey by Selvi et al. found that evening types were slightly more likely to recall their dreams overall, so this could in part explain the findings.

However, Blagrove adds that night owls who go to bed late during the week and have to wake up for work at the same time as early birds are more likely to experience sleep deficit—and to make up for it on the weekends by sleeping in. It's during this extended weekend sleep that they might experience more REM sleep, a sleep phase that is characterized by rapid eye movement, increased brain activity and vivid dreaming. "They might have a whole lot of recovery sleep on the weekend that explains why this is happening," Blagrove says.

A so-called stress hormone known as cortisol might also be involved, Payne hypothesizes. The hormone usually peaks in the body in the morning, just before we wake up. It's also around this time that REM sleep cycles also peak. "The first question is whether there is a connection between REM and cortisol peaking," Payne says. "The idea is that if your sleep has been shifted you may be asleep when cortisol is elevated, which might lead to nightmares or bizarre and vivid dreams."

It's unclear from the new study whether the students who self-reported as evening-types were true night owls or natural early birds who mischaracterized themselves because they were forcing themselves to stay up late to socialize or complete a term paper. According to Rosenberg, follow-up studies on the possible nightmare link should have participants record their habits in a sleep diary, or wear a movement-detecting device known as an actigraph unit—essentially a specialized accelerometer that can record sleep patterns based on periods of rest.

The value of knowing the causes of nightmares is not lost on researchers themselves. "I used to have a whole load of dreams ten years ago that were thrillers, with a slightly threatening aspect to them," Blagrove says. "I don't miss them at all."