I'm looking at inexpensive and un-intrusive ways to quantify the state of human body/mind . One of these is Actigraphy, which is a study of human motion over time. This can be done with an iPhone mounted on a wrist. There are algorithms that correlate actigraphy measurements to sleep and allow to calculate sleep onset latency, sleep efficiency, etc. I'm looking for additional metrics that may be added to actigraphy.
Recently, there has been a new iPhone app called "heart rate" which uses a camera to shine light through a finger, capture a series of images of the finger and count the heart rate based on the blood flowing in and out of a finger, changing the color of camera images. This is similar to pulse-oximetry derived heart rate, and maybe the technology would evolve sufficiently to give information about the blood oxygen content as well.
As a sleep researcher, I'm wandering if there are any known algorithms or studies that relate heart rate (as beats per minute) and/or pulse oximetry to sleep or human's ability to fall asleep?
It appears to me that with a device like an iPhone measuring pulse at the same time of the day, it should be possible to create a history of heart rate, while accounting for the diurnal variation of heart rate. I'm wandering if there's anything biologically or medically relevant that can be glimpsed from such history of heart rate metric.