Red light therapy is a form of photobiomodulation, and is usually referred to as non-heat producing. However, the longer wavelengths in the form of NIR will produce a certain heat development if the effect is sufficient, and it is this that allows you to experience a certain heat generation from light therapy. Even the panel itself can emit some heat due to the technical components in it, eg transformers.
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What is the difference between red light and near infrared light?
What is the difference between LED light and laser?