A lot of people don't get that much. Because of staying up late and getting up early to go to work and so forth. And so as a nation, we've sometimes been called chronically sleep-deprived. That we really don't get as much sleep as we should.
DAVID FOLK THOMAS: What are the different types of sleep?
SHELLEY ZAK, MD: Well, we generally divide sleep into two main types. REM, or rapid-eye-movement sleep, and non-REM sleep. And within the category of non-REM sleep, we further divide it in terms of depth. There's light sleep and deep sleep, and that's really, "How easy is it to rouse someone?" That's all that depth really means. You have cycles during the night. Each cycle last anywhere between an hour-and-a-half, a little under two hours, of non-REM, REM, non-REM, REM.
And, in fact, it's very common for people to wake up at the end of each cycle, after a REM period. Though most people aren't aware of the wakenings. You wake up, roll over, go right back to sleep and have no memory of it. Sometimes, though depending on circumstances, people might wake up and they'll realize, "Oh, I've been waking up after my dreams." But that actually can be normal.
DAVID FOLK THOMAS: And REM is the deeper sleep?
SHELLEY ZAK, MD: Well, it's not really deeper. It's actually totally different. Everything kind of flies out the window when talking about REM sleep. And it's actually not as deep as quote "deep sleep." Again depth is defined as, "How difficult is it to wake you up from?"