Enter the Silicon Labs "Sensor Puck" which includes a dual-mode light sensor which can measure ambient light, UV light for UV exposure accumulation, and when you place your finger over the plastic-covered LED assembly (at the bottom of the Puck), it changes modes and goes into pulse rate monitoring (ala FitBit).

https://www.silabs.com/products/sensors/Pages/environmental-biometric-sensor-puck.aspx

U5 is under a plastic shield, and it can monitor Heart Rate, and send the data out using Bluetooth BLE. I have actually been able to strap this to myself such that the window I am pointing at is close enough for the Puck to go into Heart Rate mode (J1 is a bit painful so it took some clever work with velcro strips and bandage tape).

The Puck is a bit too large to wear, but it's only a demonstration platform. Here is the back side where you see a CR2032 battery, and at the top the Bluetooth Chip:

Obviously not ideal as a purpose-built monitor! Also, the LED light/UV/Pulse sensor on the front are from various vendors and not designed by Silicon Labs.