SPOKANE — Researchers have long observed significant differences in normal people’s sleep. Some are light sleepers, whereas others sleep deeply. Some fall asleep right away, while others take their time.

Such sleeping pattern variability has long been attributed solely to differences in circumstances, habits, and other non-biological factors. But now a landmark study led by Hans Van Dongen, associate research professor and assistant director of the Sleep and Performance Research Center at Washington State University Spokane has shown that these individual variabilities constitute traits which may eventually prove genetic in origin.

The results of the research were published in the June 2007 issue of the “Journal of Sleep Research,” with WSU graduate student Adrienne Tucker as the lead author. The study assessed the presence and magnitude of trait individual differences in the structure of sleep for a group of 21 carefully screened healthy young adults, and compared these individual differences to the effect of prior sleep deprivation on the structure of sleep.

Over 12 consecutive days, study participants were monitored continuously in a strictly controlled laboratory environment. Polysomnographic recordings—which show brain waves, eye movements, and muscle tone—were done for eight nights with 12 hours time-in-bed, interspersed with three 36-hour sleep deprivation periods.

An ensemble of 18 standard sleep parameters were assessed, including sleep duration, time to fall asleep, and the amounts of the various sleep stages (stages 1 through 4 and REM sleep). Traits were quantified by means of the “intraclass correlation coefficient” (ICC), which represented the percentage of variability in the data attributable to trait individual differences. For most sleep parameters, the ICC was between 40 percent and 60 percent, indicating a moderate amount of trait individual variability.

For deep sleep (stages 3 and 4), the ICC was greater than 70 percent, indicating a substantial amount of trait individual variability.

The magnitude of the trait individual differences consistently exceeded the effect of sleep deprivation on the sleep parameters, as measured by the difference between recovery sleep after sleep deprivation and baseline sleep before sleep deprivation. So while recovery sleep structure was notably different than baseline sleep structure overall, the trait differences in sleep structure among subjects were even larger, particularly for deep sleep.

”Trait individual differences were the most dominant determinant of sleep structure in this sample of healthy young adults,” Van Dongen said.

Further research could be done to confirm Van Dongen’s suspicion that the trait individual differences in sleep structure exposed by this study may be genetic in origin.

The physiological or functional significance of these sleep traits remains a mystery. The fact that all subjects were healthy, young adults and good sleepers seems to rule out any immediate clinical relevance of the differences among them. However, Van Dongen thinks that the sleep differences may be predictive of future clinical conditions.

“Recognition of trait individual differences in sleep may help to understand the increasing evidence for a functional link between sleep and health,” he said.