That, and possibly using the light for too long on any part of your body. As well as I recall from all my research, anything over 20 min's, MAX, on any one part of the body will reduce, then totally wipe out, any beneficial effect.
That's a good point. I've never really calculated what sort of light dosage I get.
Let me calculate the actual infrared light dose I receive, and compare it to the dose that @Johannes gives himself daily (you can ignore my calculation below, and go to the final line of this post):
I place my infrared illuminator spotlight about a meter or so from me, because it creates a narrow beam (they are sold as nighttime security spotlights). So I place the illuminator at a distance so that it illuminates my whole head and torso area.
My infrared illuminator takes 3 watts electrical power. LED light efficiency is something like 30%, so we can assume about 1 watt of actual light output from my illuminator. If that light output is spread over say 0.5 square meters, then that bathes my body in around 0.4 mW/cm2 of infrared light.
Compare that to the 80 mW/cm2 that @Johannes uses for 20 minutes each day. His light is thus 200 times stronger than mine.
So in order to get the same daily light dose as he receives in 20 minutes, I would have to use my infrared illuminator for 200 times longer, that is, 200 x 20 = 4000 minutes = 66 hours.
But I probably only get around 10 hours illumination a day.
So it looks like I am getting about 7 times less of a daily infrared light dose than @Johannes. That's maybe why I did not notice much from my infrared experiments.