Fitbit may accurately track exercise and other fitness and physical activity with its wearable fitness devices, according to a new putative consumer class action, but the models purporting to measure a user’s sleep are alleged to violate California’s false advertising law.

James P. Brickman alleges that Fitbit “has made specific advertisement claims that for an extra charge, the customer can purchase a device which also contains a ‘sleep-tracking’ function which will track ‘how long you sleep,’ ‘the number of times you woke up,’ and ‘the quality of your sleep.’ In fact, the sleep-tracking function does not and cannot do these things.”

Fitbit’s sleep-tracking device advertises that it can “measure sleep quality,” with data and graphs to reveal how long a user slept and the number of times a user woke up. The functional displays purport to identify exact times with specific numbers of actual sleep time and restless minutes, among other data.

But the $30 add-on feature (found in the Fitbit Force, the Fitbit Flex, and the Fitbit Ultra, among other models) is based on “accelerometer” technology, which Brickman says a 2012 study showed fell “far below an acceptable standard of accuracy to render it useful in any way for scientific purposes.” As compared to the more accurate forms of sleep-tracking technology, polysomnography and actigraphy, Fitbit overestimates a user’s sleep time anywhere from 67 to 43 minutes, respectively.

“Despite Fitbit, Inc.’s specific representations that the Fitbit sleep-tracking function can and does track and provide precise and accurate numbers, down to the minute, of how much sleep a user gets, the Fitbit sleep-tracking function simply does not and cannot accurately provide these numbers,” Brickman, who bought a Fitbit Flex in 2013, alleges.

This “egregious overstate[ment]” of the ability of the Fitbit sleep-tracking function to perform as advertised “implicates serious health concerns,” the suit added, “as thinking you are sleeping up to 67 minutes more than you actually are can obviously cause health consequences, especially over the long term.”

For alleged violations of California’s false advertising law, unfair competition law, breach of warranty, unjust enrichment, and common law fraud, the putative nationwide class seeks restitution as well as compensatory and punitive damages.

To read the complaint in Brickman v. Fitbit, Inc., click here.

Why It Matters: In a statement, Fitbit said it intends to “vigorously defend” the lawsuit. “We do not believe this case has merit,” the company said. “Fitbit trackers are not intended to be scientific or medical devices, but are designed to provide meaningful data to our users to help them reach their health and fitness goals.” The company—which has faced criticism before about product accuracy—emphasized that the consistent tracking of an individual’s trends provides value to a consumer even if the data is not 100 percent accurate.