Leigh Honeywell, 28, of Seattle, made a resolution at the start of 2012: She decided to get more sleep. More than a year later, the computer security manager says she's keeping that resolution and owes a lot of credit to a little device she wears on a wrist strap at night.

The device is a Fitbit One, one of a growing number of gadgets consumers can buy to track their health habits, including sleep. The devices typically use movement detectors called accelerometers, which can count your steps during the day but also can detect sleep patterns by tracking your arm movements at night. You download the data to your computer or mobile device — and get multicolored charts showing how long you were in bed and how much of the time the device sensed that you were asleep or awake. At least one device, the Jawbone Up, claims to distinguish light from deep sleep.

Smartphone apps that track sleep also are proliferating. Some ask users to literally sleep with their phones so that the phones' accelerometers can detect their movements.

It is unclear how many people are using such devices and apps. A recent survey from Pew Research Center found that most people who track personal health statistics still use paper or their memory rather than a computer, smartphone or what the survey called "a medical device."

STORY: Survey: Many adults track their health issues

It's also unclear how accurate or useful the sleep data created by the trackers are. Some sleep experts are skeptical. They say, and manufacturers agree, that these trackers are no substitute for medical advice or lab testing when people have serious sleep problems.

But Honeywell, who used an app before getting her Fitbit, is convinced they have made a difference in her once sleep-deprived life.

Seeing her sleep data, she says, convinced her that "there was a pattern," and it was not healthy: "I would get progressively less sleep for about 10 days in a row, and then I would crash and catch up for a few days, then do it all again," she says. She also realized she craved candy on the days she got the least sleep.

Now she's consistently sleeping eight hours or more each night, feeling more alert and resisting candy, she says. Honeywell says she got exactly what the device makers say they intend to provide — insights that can change habits for the better.

"It's hard to get better if you don't know where you are starting from," says Travis Bogard, Jawbone's vice president of product management and strategy. Many people, he says, quickly find there's "a huge gap between intentions and actual behavior."

A sleep tracker that is also an activity and calorie tracker lets people see relationships between sleep, eating, exercise and weight, says John Stivoric, chief technology officer of BodyMedia. The company makes sleep- and activity-tracking armbands used by contestants on NBC's The Biggest Loser. Combining the data, he says, can "help give people a sense of how sleep is affecting their journey" to weight loss or other health goals.

Some people who start tracking sleep do notice worrisome patterns — such as waking dozens of time a night — and decide to check them out with doctors, says Eric Friedman, Fitbit's chief technology officer. He says one user who saw that he was waking up 83 times a night went to a doctor and found out he had sleep apnea, a serious breathing disorder. "We hear those stories all the time," Friedman says.

But none of the products is designed to tell people what's normal — to say how many times they should wake up each night or what constitutes a good "sleep efficiency" score (the percentage of in-bed time you are asleep, provided by several of the devices). Instead, users are invited to compare their numbers with one another and to consider their own data over time.

That data could be misleading, says Hawley Montgomery-Downs, a sleep researcher at West Virginia University. She is one of the few independent researchers who have looked at any of the new devices, and in a study published in 2011, she found that a previous version of the Fitbit was not only less accurate than polysomnography, the sophisticated gold-standard sleep lab test, but also less accurate than a device called an actigraph – a watch-like accelerometer that is sometimes used in sleep studies and was the inspiration for the consumer devices. In her study of 24 healthy adults, both Fitbit and the actigraph overestimated overnight sleep time compared with the lab test, Fitbit by 67 minutes and the actigraph by 43 minutes.

In general, she says, accelerometers are more likely to say people are asleep when they are awake than the other way around — meaning they might give people with sleep disorders an overly rosy view. "That could be dangerous," she says.

Matt Bianchi, a sleep specialist and assistant professor of neurology at Harvard Medical School, agrees. "My worry is that the people who are buying these things might tend to have real sleep problems and might be falsely reassured by a device that seems to say they are fine."

He says there's also no evidence an accelerometer can tell light from deep sleep. Jawbone's Bogard says the measure used on his company's device "doesn't directly map" the formal stages of sleep recognized by sleep doctors but does track "restfulness" — how still or active a sleeper is.

Montgomery-Downs and Bianchi say they have no problem if the devices are used to raise sleep awareness among healthy people, as long as they know the limitations.

Russell Sanna, executive director of the division of sleep medicine at Harvard Medical School, is more enthusiastic. Tracking sleep, even with imperfect devices, could be a great way for more people to realize how important shut-eye is and how it affects everything from work to appetite, he says.

Sanna says he uses one of the commercial sleep trackers (he won't say which one) and "I love it."

Consumers who want to try a sleep tracker that can download data to computers or mobile devices have plenty of options. Among those that use accelerometers to track sleep and other health stats:

• Fitbit One: The device clips onto clothing during the day, goes on a wrist strap at night. A day-and-night wristband version, the Flex Wireless, will come out in April. $100. www.fitbit.com

• Jawbone Up: The rubber wristband comes in many colors. $130. www.jawbone.com

• Larklife: The rubber wristband comes with a softer fabric sleep bracelet. $150. www.lark.com

• BodyMedia armbands: The Link ($150) and Core ($120) bands strap to the upper arm and add temperature and sweat sensors to movement detection. www.bodymedia.com

• Basis: The watch-style device also uses multiple sensors and comes with black and white bands. $200. www.mybasis.com

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