The new SpO2 sensor also measures the changes in your blood oxygen levels, and Fitbit uses a proprietary algorithm to calculate these factors into an overarching Sleep Score.

The Charge 4 uses a three-axis accelerometer and the optical heart rate sensor to measure your time asleep and the amount of time you spend in different sleep stages. On the plus side, Premium is still a deal if you were planning on enrolling in a separate coaching or training program anyway. It does stink a little that Fitbit hides so many of its best software features, like the advanced sleep software, guided programs, and personalized insights, in the Premium subscription. Its sleep features are the best in the market, and one of the reasons why you should probably spring for a Fitbit Premium membership, which costs $10 per month or $80 per year. Longer battery life means you can wear the watch while you’re sleeping, another arena where Fitbit excels. It's a relief to cut one device from my nightly routine of plugging in my phone, laptop, and kid's iPod.

When tracking one to two activities per day, with notifications turned on and at normal brightness, the Charge 4 lasts me between 5 to 7 days. It is so, so nice to not have to charge it every night like I have to do with my Apple Watch.

To me, the primary reason to get a Fitbit is battery life.

Fit bit charge 4