The firm's Ionic, Charge 3 and other Versa fitness tracks now have the brand new health tool which studies SpO2 levels.
TizenHelp reports that Fitbit added graphs which show the difference in oxygen levels from when someone went to sleep and when they wake up.
The Apple Watch was reported in 2015 to have the feature, but it has laid idol on their devices ever since.
The SpO2 sensor can also help detect other sleep-releated issues such as sleep apnea.
Meanwhile, in November it was confirmed that Google was buying Fitbit for £1.6 billion.
The company explained that they saw the tech giant as "an ideal partner" to expand after the business was making a loss and struggling with waning demand with other companies entering the market.
Co-founder and chief executive James Park said: "Google is an ideal partner to advance our mission.
"With Google's resources and global platform, Fitbit will be able to accelerate innovation in the wearables category, scale faster and make health even more accessible to everyone."
Fitbit - which was founded in 2007 - insisted its "health and wellness" data wouldn't be used by Google adverts amid concerns from regulators, and the company also vowed to continue its strong privacy protections.
The acquisition is expected to be confirmed and completed in 2020.