To elaborate further: As you can see by looking at my stats, I’m probably one of the people who have used LingQ the most in the last years. Although I think I can say I have experienced some sorts of burnouts from using it, my experience is different from yours. I was never forcing myself to learn while having headaches and feeling dizzy or nauseous or anything like that. It’s more that I at times just wouldn’t be receptible to learning anymore. I’d try to read materials and listen to them but not be able to focus and zone out, so I’d give up. I also did have times where other aspects of my life suffered due to all the time I spent learning on LingQ, getting Icelandic into LingQ and just being in front of a computer screen in general and it would start making me unhappy. When that happened, I’d try to make sure I’d get more exercise and real life social interactions.
What got me a lot closer to burnout than ever studying languages on LingQ was getting Icelandic added and adding and creating all the material for the library. I ended up setting certain deadlines where I would plan to take breaks from it at certain points.