You really should go to a doctor. Like. really. If it's common and you don't have a clear "its this" (like standing up suddenly, or some other condition you know about,) Sudden lightedness that comes and goes could be one of several awful things.
(or it could totally something not scary at all.)
We're not doctors... (and honestly, it's not stupid to ask a doctor this. but it is kinda stupid to ask randos on the internet. thats how you wind up snorting ground up leeches or something weird.)
Absolutely. My sister had brain cancer. The symptom that sent her to a doctor was "absence seizures", where she would sit quietly, apparently awake, but unable to respond to anyone speaking to her. She said it was a very strange feeling, being aware but not really "there". She died six months after diagnosis.
Is it recent and only with some TV's or maybe with a new one you bought. If so its possible changing some settings will make it more palatable. Apologies but I really don't know about this stuff as I have not messed around with configs but I have seen a variety of stuff around modern tvs and settings. In particular I found out why my work tv gave me a creep uncanny valley sensation compared to my home one.
yeah sorry. again its not like I changed the settings so I don't remember the terms although that brought it back. It was just a learning moment on how screens today can have some wack settings options.
Q: physically, how are you watching them? On a couch? Lying in bed? Hunched over on the bus? Phone? TV? Do you have auto motion plus enabled (you monster)? Do you watch it in one place? Has that room been checked for carbon monoxide? Is it a basement? Has it been checked for radon?
It could be a lot of things and there's not enough information provided but the first thing that occurred to me is blood pressure. Do you have a blood pressure cuff? Try to use it when you're feeling light headed and when you're not and comparing numbers.
It could be a physical things about the position of your body, pressure on something because you're hunched over or something. It could be a visual things too. There are conditions where moving/changing visuals could affect you.
It's kind of overwhelming all of the things it could be and you really should go to a doctor.
While you're waiting for your appointment try to make observations about what is the same about reading vs watching and what is different. This will help them figure out what to investigate.
But it might be worth buying a machine to check your blood pressure (they're not terribly expensive) because it's hard to capture it in the doctor's office. Take a measurement or three before you feel light-headed, keep the cuff on and take more measurements when you start to feel light-headed. Keep a record of the measurements (and how you were feeling at the time) to help your doctor rule blood pressure in or out as a cause.
Need more details. What are you watching when it happens? Or are you saying if you watch CSI, Sesame Street and a David Attenborough documentary about ants back to back you'd get light-headed 3 times, once during each?
As ValiantDust said it could just be motion sickness depending what it is you're watching.