Blood oxygen levels (SpO2 and SmO2) are useful in many physiological studies. The technology is now well proven and works by shining more than one wavelength of light into the skin and then measuring the amount of light that has been absorbed at each wavelength. Since blood cells absorb certain wavelengths (according to their state or saturation) more than others then it is possible to not only see the oxygen levels of blood (as well as other levels) but also heart rate can be derived. This sort of technology is now so common that watches (Apple watch etc) provide this data. Not all devices are created equal though, so caution is needed with some (sometimes cheaper) devices. In particular, some devices have very long time-lags and do not react quickly to step changes (this is how they create smooth data).