Who needs a better night's sleep? Fitbit’s betting that you do.

The fitness tracker company on Monday announced new tracking tools designed to give users greater insights into not just how much sleep they’re getting, but also how restful that sleep is.

Fitbit worked with sleep scientists to develop the software, but experts who have expressed concern about the accuracy of sleep-tracking fitness devices in the past tell Consumer Reports they want to see more research on Fitbit's new technology before coming to any firm conclusions about its usefulness.

The company's sleep-tracking tools, which will roll out in late March as upgrades to the Fitbit mobile app, were introduced alongside a new fitness tracker—the Alta HR, which tucks a heart-rate monitor into the ultraslim profile of the Fitbit Alta. The new device, which will sell for $149.95 starting in April, touts up to 7 days' worth of battery life. That's equal to the staying power of the Fitbit Surge that sits atop our ratings.

Fitbit's Alta HR fitness trackers in all seven colors

Fitbit says the first sleep tool, “Sleep Stages,” will not just tell users how much time they slept, but also break down how much of that time was spent in light sleep, deep sleep, and REM sleep, the latter of which is closely associated with dreaming and memory formation.

The second tool, “Sleep Insights,” takes the data collected by a user’s device, compares it with data gleaned from 3 billion nights' worth of Fitbit users' sleep, and provides individualized recommendations. For instance, the app may notice that a user is sleeping much longer on the weekend and let the person know that the lengthy slumber could be a sign that he or she needs more sleep during the workweek.

“It’s not a one-size-fits-all approach,” Dr. Allison Siebern, a consulting assistant professor at Stanford University’s sleep center, said in a phone interview. “It’s a great tool for helping people figure out what their personal sleep needs are—what works for them, what doesn’t.”

Siebern, Dr. Michael Grandner of the University of Arizona, and Dr. Michael T. Smith, Jr., of Johns Hopkins University all worked as paid consultants on the project, sharing sleep expertise with Fitbit’s research and development team.

But many questions remain about the efficacy of such sleep-tracking tools. According to a February study published in the Journal of Clinical Sleep Medicine, "most consumers are unaware that the claims of these devices often outweigh the science to support them as devices to measure and improve sleep."

The authors criticized fitness trackers for depending too heavily on users' movements to determine sleep levels, for a lack of transparency in their sleep-tracking algorithms, and for a tendency to reinforce bad sleep habits by encouraging extended time in bed.

The lack of transparency surrounding the algorithms used to calculate sleep make it “impossible to know how accurate they are even under the best circumstances,” the study says.

When asked about the study cited in JCSM, Fitbit pointed to two other studies (abstracted here and here) that show that fitness trackers can be effective in tracking sleep. In a statement, it explained that its algorithm is the product of "extensive internal testing" that involved the analysis of several hundred nights of heart-rate and movement data from volunteers whose sleep patterns were observed and mapped with clinical-grade equipment.

Fitbit's R&D team also factored other data about the influence of sleep stages on heart rate and movement into the algorithm.

The University of Washington's Dr. Nathaniel Watson, a board-certified neurologist and sleep specialist, said fitness trackers are good at encouraging people to pay attention to the quality and the amount of sleep they’re getting.

But he echoed the concerns voiced in the JCSM study, saying he's not convinced that Fitbit has succeeded in solving the device's precision problems. While it’s plausible that a company could design a consumer device that accurately tracks the stages of sleep by analyzing heart rate and movement, he explained, he has yet to see one that does so.

“The onus is on them to do the research to support their claims,” he added. “I think the medical community will be interested to see that data.”