4 Smartwatch Features That Track Your Overall Health

3. Sleep tracking. Some devices, including high-end Fitbits and Galaxy smartwatches, track the quality of your slumber time right down to various sleep stages and the number of times you wake up in the middle of the night. Apple’s watch is more about setting up the conditions so that you get a good night’s sleep, starting with a wind-down routine before bedtime.

Of course, if you’re planning to sleep with any of the smartwatches, make sure they are charged before you go to bed, and if needed, give them some extra juice when you wake up. Only the Fitbit Sense had longer than a 24-hour day’s battery life, in what PC Magazine considered normal use.

4. Blood oxygen. Fitbit, Garmin, Mobvoi, Samsung and now Apple’s Series 6 can all measure blood-oxygen levels. Series 6 obtains a measurement from a quartet of clusters of green, red and infrared LEDs on its rear and four photodiodes spaced and isolated between them to determine the color of your blood.

Measurements are automatically collected throughout the day or when you’re asleep. You also can launch an app to take a manual reading by steadying your wrist on a table, with the watch display facing upward, tapping on Start, and then waiting patiently while a timer counts down for 15 seconds.

While most healthy adults report levels between 95 percent and 100 percent, what does a lower blood-oxygen reading signify? Blood oxygen is an indicator of early signs of circulatory, heart or lung function issues, such as anemia, neurological problems or sleep disorders, says Leslie Saxon, M.D., a professor of medicine and executive director at the Center for Body Computing at the University of Southern California.