Recently, we’ve been hearing reports that the Apple Watch’s plethysmograph sensor plays not-so-nicely with wrist or arm sleeve tattoos. The ink pigmentation interferes with the sensor’s ability to read your heart rate — and with it, the Watch’s ability to assess whether or not it’s maintaining skin contact.
Apple uses various spectrums of light to track the blood flow through your skin. Anything that reduces that light’s reflectiveness — ink pigmentation within your skin, for example — can interfere with that sensor.
For those who have similar issues with their Apple Watch, you can turn off Wrist Detection in the Apple Watch app to avoid the device auto-locking; unfortunately, that will also disable Apple Pay usage. Alternatively, you might try wearing it on a non-tattooed wrist.