Last Updated May 13, 2011 9:52 AM EDT
Q: Why do studies say we are "not at our best" when we don't get 7 or 8 hours of sleep, yet other studies show we need less sleep the older we get?
Van Dongen: Older individuals typically sleep less, but there is no evidence that they need less sleep to function optimally. However, they do suffer the consequences of sleep loss less substantially than younger adults and adolescents. In other words, if younger individuals get insufficient sleep, it has a greater impact on them.
Q: Do naps prevent the mental decline in people who get 6 hours of sleep?
Van Dongen: Naps are a great countermeasure for insufficient sleep. As a rule of thumb, naps make up for lost sleep on an hour-for-hour basis. Thus, 6 hours of sleep plus a 2-hour nap is about equivalent to 8 hours of consolidated sleep.
Q: Several BNET commenters questioned the research that shows that people need 7-8 hours of sleep. They say they do well on 5 or 6. Can you respond to this?
Van Dongen: Chronic sleep restriction leads to poor self-evaluation of performance capability. In other words, chronically sleep-deprived individuals are not in the best position to reliably evaluate their own ability to deal with the sleep loss. It is true that there are some individuals who really need much less than 8 hours per day and can still function optimally, but such individuals are rare and usually they do not accurately self-identify.
Q: In the studies, people were likely woken up in the middle of their 90-minute sleep cycle. Wouldn't this contribute to their sleepiness more than how much sleep they got?
Van Dongen: People waking up from "slow-wave sleep" or deep sleep, the first part of the 90-minute cycle (which is actually about 100 minutes long for the average person), experience a phenomenon called sleep inertia (disorientation, performance impairment and tendency to fall asleep again). People waking up from REM sleep, the other part of the sleep cycle, do not normally experience much sleep inertia. Natural awakening is typically out of REM sleep, as it is not as deep. Sleep inertia from awakening out of deep sleep normally dissipates in less than an hour, fortunately, with most of the effects gone in about 15 minutes. (In our study, sleep inertia was controlled for so it does not explain the effects of sleep restriction we observed.)
Q: How significantly does sharing a bed (partner, kids, etc) compromise sleep quality and restedness?
Van Dongen: Sharing a bed partner does not necessarily have a significant effect on sleep. However, a restless or snoring partner can disturb sleep significantly. If this is an enduring and significant problem (e.g., if the partner is gasping for air during the night or kicking his or her legs), it may be a good idea for the partner to be evaluated for a possible sleep disorder (such as sleep apnea or restless legs syndrome) and treated if necessary.
Any other sleep questions? Post them below, and I'll try to answer them.