Stress Check: Measuring your stress level with your iPhone or Android smartphone

The “Stress Check” application for iPhone and Android makes use of your phone’s camera and LED flash to measure heart rate variability and hence your stress level.

The “Stress Check” smartphone application promises to take measurements of your body’s psychological and physical stress levels. Available on the iPhone and Android platforms, a quick 2-minute test using the application will give you an immediate assessment of your current stress levels. This knowledge can help to diagnose the environmental factors contributing to your stress levels and could help you to reduce or eliminate those factors from your daily routine.

“Stress Check” works by making use of your smartphone’s camera to measure “heart rate variability” – a key indicator of stress. Additionally, your smartphone’s LED flash is used as a source of illumination and to make heart rate measurement possible.

How does a smartphone measure heart rate?

About a year ago, we covered the ‘Instant Heart Rate’ application for iPhone and Android smartphones. ‘Instant Heart Rate’ was developed by the same company as ‘Stress Check’ with both applications using the “pulse oximeter” technique to measure your heart rate.

Pulse oximeter applications such as ‘Instant Heart Rate’ and ‘Stress Check’ are able to take measurements of your heart rate by making use of the camera on your smartphone to observe small changes in your skin colour. The phone’s LED flash is used to illuminate your finger which should be lightly be placed over the camera lens.

Each time your heart beats and oxygen is pumped around the body, your blood becomes more oxygen rich and will absorb more light. This affects the amount of light which is reflected by the finger back into the camera. These small changes in skin colour are measured by the application and are used to determine heart rate. It’s a very similar technique to that used in medical pulse oximeters (though pulse oximeters typically use transmittance through the finger rather than reflectance from the finger).

How does my smartphone measure heart rate variability (HRV)?

Unlike ‘Instant Heart Rate’ which gives an instantaneous measurement of your heart rate, ‘Stress Check’ takes things one step further by looking at how your heart rate varies over time. It is well known by scientists that the time between two successive heart beats is not always the same: for example there could be 0.8 seconds between two heart beats, then 1.1 second until the heart beats again. Measuring the time interval between successive heart beats and looking at how this interval changes over a period of a few minutes will allow heart rate variability (or HRV) to be measured.

Various studies have been carried out into the relationship between heart rate variability and stress levels. The key conclusions of the research are that a high heart rate variability would indicate a state of relaxation or calm whereas a low heart rate variability would indicate high stress levels. It is by using this relationship that the app is able to determine your stress level from measurements of your heart rate.

How accurate is this application?

‘Stress Check’ hasn’t been clinically tested so we certainly wouldn’t rely on it for important medical measurements. The reliability of the application could also depend on the exact phone you use (for example, smartphones differ in how far the LED flash is placed from the camera lens). Anecdotally, during our testing, we felt the results from ‘Stress Check’ seem to match up fairly well with our own perceptions of stress.

According to Azumio, the measurement results should be fairly robust as they use the same algorithms which are used in professional devices:

Algorithms used to analyze HRV follow recommendations of European Society of Cardiology (ESC) and the North American Society of Pacing and Electrophysiology (NASPE)… Stress Check utilizes a method of quantitative stress level assessment developed by Biocom Technologies, a well-known developer of various professional health assessment technologies.

As a slight aside, we are slightly wary as to how accurately the application can measure stress level. For example, what is the meaning of percentage stress level? What does it mean to be 50% stressed or 100% stressed?

Is this a practical application which can be used on a daily basis?

The benefit of having a stress checker application built into your smartphone is that the convenience of using it makes it possible to diagnose which factors could be causing stress in your daily life. For example, could your stress levels be linked to your morning commute or do they alternatively result from the barrage of e-mails arriving through the day on your smartphone? By varying these factors in your daily life and using the ‘Stress Check’ application, it should be possible to pinpoint the causes of stress.

The key downsides to the application are that it takes around 2 minutes to take a measurement of heart rate variability (HRV) and stress level. Furthermore, the LED flash must be on during this entire time so the application will quickly eat up battery.

Where can I download the ‘Stress Check’ application?

‘Stress Check’ is available for the iPhone from the iTunes App Store (69p download) or alternatively on Android from the Android Market (free download).