Apple Watch and Tattoos ➝

From an Apple Watch support document:

Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.

If you have tattoos on your wrist and have been trouble getting consistent heart rate readings, this could be why.

(Via 9 to 5 Mac.)