My own pattern of sleep seems to be slightly off-kilter. I get plenty of REM sleep--about 25 percent--but not enough deep sleep--about 10 percent, rather than the average 20 percent. Deep- or slow-wave sleep--the stage of sleep that is most difficult to wake from--has been linked to some types of memory, and a recent study suggests that lack of deep sleep can affect insulin sensitivity, a risk factor for type 2 diabetes.

An evening beer didn't seem to have any effect on my sleep patterns, but a benzodiazepine sleeping pill did. As predicted by previous research, it dropped my slow-wave sleep percentage further, to about 5 percent. "Sleep is much more active and dynamic than people perceive," says Shambroom. "This allows you to understand your personal sensitivity to caffeine and alcohol." (As a highly caffeine-sensitive person, I didn't risk a late afternoon latte, even for the sake of science.)

What exactly this means for me is unclear. I'll probably stay away from that kind of sleeping pill. But the experts I spoke with were hesitant to comment on what my broader sleep patterns meant for my overall health, probably because the purpose of different types of sleep--indeed, the purpose of sleep in general--is still hotly contested. (There are few ways to increase slow-wave sleep specifically; increasing body temperature prior to the sleep cycle is one.) Most of the advice from Zeo centers on well-known recommendations for "sleep hygiene," such as avoiding caffeine, alcohol, and stimulating activities right before going to sleep. (Disclaimer: I used the device only for a few nights, not long enough to test the online coaching program that comes with it. This program sends users e-mails with advice based on their individual sleep data.) "Really, the litmus test is how you feel during the day," says Northwestern's Zee.

As noted earlier, Zeo emphasizes that the device is not intended to be used as a medical device. It cannot, for example, diagnose sleep apnea, the most common sleep disorder, or disturbances such as periodic limb movements and seizures during sleep. But some worry that this warning might not be heeded. "If you have conventional insomnia, the Zeo would be fine," says Jerome Siegel, director of the sleep research lab at the University of California, Los Angeles. "It would confirm you have insomnia and provide a way of quantifying and validating your sleep patterns as you do the conventional things to treat it." But the more general issue, he says, "is that people who have the most severe sleep disorders won't detect [them] with this device and then they may not go to their physician to be diagnosed."

Siegel and others, however, are interested in the Zeo's potential as a research tool. While sleep scientists have investigated numerous aspects of sleep--including the effects of drugs, age, and diseases such as depression--these studies are enormously expensive (a night in a sleep lab costs about $1,000). A relatively cheap home-use device would allow researchers to look at the variability of normal sleep patterns in a much larger number of people. "It's simple to use, practical, and it can be monitored on a daily basis," says Zee. "That has been a limitation in the field--recording sleep day after day."

While a sleep lab generates much more detailed information, the Zeo might provide a way to study normative sleep patterns, such as variation with the season, adds Siegel. "It's extremely interesting," he says. "All sorts of things that would have been prohibitively expensive now become quite practical."