Press Release

The ability to remain awake and alert in quiet situations may be related not only to sleeping patterns, but also to the physiologic features of heart rate and heart rate power, according to a study in the June 1 issue of the journal Sleep .

Results show that members of a study group with both objective and subjective levels of sleepiness have a slower average heart rate than both alert subjects and a group of participants who are objectively sleepy but feel alert. Both groups of sleepy subjects also show a smaller average ratio of low to high spectral heart rate power than alert subjects.

Michael H. Bonnet, PhD of the Dayton Department of Veteran’s Affairs Medical Center in Dayton, Ohio, and co-author Donna L. Arand, PhD, sought to determine the relationship between objective and subjective sleepiness scores across several nights. Their results emphasize the importance of the link between cardiac function and sleepiness.

“Sleep researchers and the public need to understand that sleepiness is controlled by more than total hours of sleep,” says Bonnet. “Understanding sleepiness also requires an understanding of arousal. Just as some people are always going to be short, some people will always have low levels of arousal regardless of their sleep.”

Correlations between objective Multiple Sleep Latency Test (MSLT) results and subjective measures of sleepiness can be low, according to background information in the article. Many people defined by an MSLT as being sleepy, meaning that they fall asleep in less than seven minutes in a quiet daytime setting, report feeling a normal level of daytime alertness.

The study shows that these people who feel alert but fall asleep quickly have a faster heart rate than the subjectively sleepy group. Yet while they feel alert, the data show that they do exhibit traits of sleepiness. Their reaction time as measured by a 10-minute simple reaction time task is lower than that of the alert study group.

Results for participants who exhibit subjective and/or objective sleepiness show normal sleeping patterns with an average total sleep time of more than seven hours at night. According to the authors, this suggests that young adults who sleep normally can still fall asleep quickly even in the absence of sleep deprivation.

The study involved 50 healthy men and women, between 18 and 39 years of age, who have normal sleep patterns. Thirty-six of the participants were selected for comparison. Eleven were defined by MSLT as being alert, 12 were defined as sleepy but felt alert, and 13 were defined by MSLT as sleepy and also felt sleepy.

Their sleep was monitored in a sleep laboratory by polysomnogram at night and by MSLT during the day. Subjective sleepiness was measured by their responses to the Stanford Sleep Questionnaire and Assessment of Wakefulness. Metabolic observation collected heart-rate data, and psychomotor tests were used to measure alertness.

This study was supported by the Dayton Department of Veterans Affairs Medical Center, Wright State University School of Medicine, and the Sleep-Wake Disorders Research Institute.

The journal Sleep is the official publication of the Associated Professional Sleep Societies, LLC, a joint venture of the American Academy of Sleep Medicine and the Sleep Research Society. It is a peer-reviewed research and clinical journal addressing sleep, circadian rhythms, and the diagnosis and treatment of the broad spectrum of sleep disorders.