Fitbit has recently released its follow-up fitness tracker for Fitbit Charge 2, the new Alta HR, which is tagged as the "world's slimmest fitness wristband." This new fitness tracker also introduces the sleep feature called "Sleep Stages," while it houses tracking feature for distance and calories burned and displays notifications.

Following the successful release of Fitbit Charge 2, Fitbit has continued to improve its fitness tracker's capabilities and design with the launch of another device under the Alta family. The new Alta HR features some features that are resident on the original Alta device, although it introduces a slimmer design, added functions such as the "Sleep Stages" feature and continuous heart rate monitoring, and longer battery life.

Among the array of features included in Fitbit's Alta HR are the company's signature activity recognition and sleep tracking, support for notifications such as incoming calls, text messages and calendar alerts, and a large battery, all shown in a 0.6" wide LCD display.

In the new Alta HR fitness tracker, Fitbit is branding a slim design that will compete as the world's slimmest fitness tracker. This smaller chassis was achieved through Pure Pulse's improved chip that reduces the size and number of components needed by 25 percent, Mirror.co.uk reported.

Another highlight in the Alta HR is its sleep feature called "Sleep Stages." The company claims that the new feature can break sleep down in stages using continuous heart rate monitoring through Fitbit's PurePulse technology, CNET reported. Through the optical heart-rate sensor which automatically tracks the user's heart rate throughout the day, Alta HR can estimate how long users spend in light, deep and REM sleep cycles, including sudden time wakes in night.

"Fitbit's new sleep features use a scientific-based approach to show your sleep patterns over time, and provide you with validated, actionable guidance to help you make changes in your daily routine," said Dr. Allison Siebern, consulting assistant professor at Stanford University's Sleep Medicine Centre.

"Given the comfort and accessibility of this product, it's one of the most valuable and useful sleep tracking solutions available to consumers outside of a sleep lab," Siebern added.

The "Sleep Stages" function will debut in Alta HR band, but will also be coming to the Charge 2 and Blaze bands through a software update.

The Alta HR can already be purchased on Fitbit.com for £129.99. Starting in March 13, Alta HR will be available online and in stores, including major retailers such as Amazon.com, Argos, Currys PC World, Shop Direct and John Lewis.

Sleep Stages and Sleep Insights will be slated for global release on Android, iOS and Windows from March 27.