I only know it because it says it on the instructions in the pack but... I did find this on the MD website
"Patch warning. Direct sunlight or high heat can increase, then decrease, the amount of hormone released from a patch. This can give you a big dose at the time and leave less hormone for the patch to release later in the week. Avoid direct sunlight on the hormone patch. Also avoid using a tanning bed, heating pad, electric blanket, hot tub, or sauna while you are using a hormone patch."
Taz x