The Basis B1 Band, an activity-tracking watch that also reads heart rate and skin temperature, got a big update today, with a new sleep-tracking feature that measures deep sleep, light sleep, and REM cycles. I've been wearing the device to bed for four or five days with a pre-release version of the new feature installed to see how well it works.

The watch has long measured sleep, but it was basic at first: how long were you still, and how many times did you toss and turn? The new sleep features don't just look at movement but take into consideration heart rate, skin temperature, and other data that the watch collects to make intelligent guesses about your true sleep cycles.

Nighty Night!

Comfort remains the main obstacle, as the Basis B1 is a full-sized wristwatch that you have to wear a little snug in order for the sensors on the back to make contact with your skin. Six raised metal bumps measure temperature, and an optical sensor reads heart rate by measuring the volume of blood flowing through your veins.

The first few times I wore an activity tracker to bed, I was absolutely cognizant of it and slightly distracted by it, but it improved with time. Still, when I wear a tracker to bed, I occasionally wake up and notice that something's pressing into my wrist. Of all the devices I've tested that track sleep, the Basis B1 is the clunkiest by far.  By day, it's no more distracting than any other hefty wristwatch, but in bed, it feels out of place.

When you fall asleep, you don't have to do anything with the Basis B1 Band, other than wear it. With the Fitbit Force and Fitbit One, you have to start recording sleep or wait until the next morning and log your bedtime and rising time in the Fitbit app. The Misfit Shine also requires starting a sleep mode, whereas the Basis figures out when you fall asleep automatically.

In the morning, you can sync the Basis watch to the companion app, called MyBasis, but it won't tell you your sleep analysis right away. It says you have to be awake at least 30 minutes before it knows for sure you're not still snoozing that alarm clock.

When your sleep data finally does appear in the MyBasis app, it shows deep sleep, light sleep, and REM sleep on a color-coded graph. You can also see the times when you tossed and turned, and when you actually got up to drink water, use the bathroom, or whatever other reason might pull you out from the covers.

The more you wear the Basis B1 to bed, the more it learns about your habits. It will eventually establish averages for how long you sleep and how much time you spend in different sleep zones. Cards showing these kinds of details shed a little more light on your patterns.

Slipups and Hiccups

The watch's sleep-tracking isn't foolproof. Just last night, for instance, I went to the movies. While sitting still for nearly two hours, the Basis thought I got a rather restful 8 minutes of deep sleep and 25 minutes of REM sleep.

Now, I'm not saying my head didn't bob once or twice as I started to nod off during the film (hey, it happens), but I certainly didn't sleep. But some combination of stillness and a slowed heart rate or change in skin temperature indicated that I was out cold, even though I wasn't.

Sometimes, and with the Basis B1 Band in particular, I hear from readers who take issue with these kinds of errors. Because the device costs $199, consumers have high expectations for the Basis B1 Band. They want it to work flawlessly. I don't blame them, but I also have much more realistic expectations having tested somewhere on the order of dozen activity trackers and fitness devices. I see what the Basis B1 Band can do that the others can't do at all, and therefore I'm more forgiving of its hiccups and slipups.

What Can The Data Do For You?

Whether the data is perfectly accurate—and whether you need it to—all depends on how you act on it. Data is only as valuable as whatever you do with it. In my case, I sleep well. I have all my life. Both the Basis B1 Band and Fitbit devices that track sleep have confirmed as much. There's not a whole lot more I could possibly do with that knowledge.

My colleague Brian Westover, on the other hand, used to sleep terribly. Then he tested the Zeo Sleep Manager and discovered unusual patterns in his restfulness. It turned out that a train near his home was disrupting his sleep at the same intervals every single night. He started wearing earplugs to see if that would help, and he now says he gets a whole heck of a lot more sleep.

In looking for patterns, it does help to see the detailed sleep cycle that the Basis band shows: REM, deep sleep, light sleep. If you're waking each night out of a deep sleep at roughly the same time, there's definitely a trigger, and you'd want to identify it. If you routinely wake up during a light sleep cycle, however, it's harder to know whether there's an external cause to blame. Some people just wake up throughout the night (I like to joke that sleep trackers can't cure a guilty conscience), and those people may need to look for other sources of their restlessness, like not getting enough exercise, which the Basis B1 Band can help to identify as well.

Sleep tracking isn't going anywhere. I think it will be as hot in terms of trends in fitness trackers as heart rate monitoring. And I think the Basis B1 Band is paving the way for other devices with this new implementation for deep sleep tracking. It will get incrementally better as other players in the market try it, and as Basis Science learns from its users what works and what doesn't. And we can all rest easier knowing that much is true.