Study Shows Subjects Overestimate Sleep Time

How long do you sleep every night? Probably not as much as you think you do, according to a new study published in this week's issue of the Journal of Clinical Sleep Medicine.

In "Relationship Between Reported and Measured Sleep Times: The Sleep Heart Health Study (SHHS)," researcher Graciela E. Silva of Arizona State University reports that sleep study subjects typically overestimate how much they sleep. While subjects gave more accurate estimates on the morning after taking a sleep test, they still tended to guess they had slept longer than they actually did, the study found.

A strange thing happened, though, when Silva analyzed subjects for how long it took them to fall asleep. The morning after a sleep test, subjects overestimated by five minutes how long it had taken for them to nod off. Normally, however, their estimates of how long it took to fall asleep were almost identical to the actual amount of time taken.

Silva's study looked at the sleep habits and estimates of 2,113 people over the age of 40. More than half (53 percent) of the subjects were women, 75 percent were Caucasian and 38 percent were obese. Adjusting for demographic differences, Silva concluded that people regularly overestimated how much they slept by 61 minutes. On the morning after a sleep test, their estimates were still high, but only by 18 minutes.

Both obese people and those with higher educations tended to report less sleep time than others, Silva found. They also differed from others in "small but significant" amounts in how long it took for them to fall asleep.

"The findings from this study suggest that results from studies subjectively assessing sleep times may not be comparable to those using objective determinations," Silva said.