I believe it'd be useful information to have a long-term log of heartrate. One example of such a device is this wearable pulse oximeter. Its $500 and records 80 hours of data.

A simpler version (no oximeter, just pulse) I believe could be done for very little cost. Using the remarkable teensy microcontroller board, a light, and a lightsensor, I am to make a wearable long-term heartrate monitor for myself and the Portland Quantified Self crowd. The parts should be a $19 teensy, a $5 battery, a $3 sensor, and a $1 led.

A video is here that shows the teensy and what the sensor looks like. Its currently measuring the amount of light hitting the sensor and displaying a relative value.

Challenges are

• making the sensor, LED, teensy, and battery (not shown) easy to wear.
• the algorithm to detect a heartbeat from the change in light intensity