Impact of the Light from a Television
I am not talking about light as in enlightenment coming from the television, but simply the light itself that is emitted from the television screen.
If you stand in a room outside of the room where the television is playing, you will see the brightness of the room constantly changing from dim to bright, from very dim to very bright, and so on.
When the television is playing in the evening hours, then it can become a light of sorts for that room.
Now think about your eyes. When there is more light available, the opening of your pupils will tend to get smaller and when there is less light available, the opening of your pupils will tend to get larger.
Imagine your eyes in a television environment in the evening.
Because it is evening, there is less light available, even with the television turned on, so your pupils are open wider to allow enough light to enter so that you can adequately see.
However, because you are looking directly at the television, your pupils tend to get smaller.
In this situation, a question arises as to what becomes more influential on your eyes, the darkness of the room, or the brightness of the television.
If the darkness of the room takes the dominant effect, then your eyes, or pupils, being wide open, may be letting too much light into them as they focus on the lit television screen. Think about the drastic impact on your eyes when you go from a dimly lit room, like a movie theater, out into the daylight of a sunny afternoon. We squint our eyes and reach for our sunglasses to shield and protect our eyes from this sudden change.
If the brightness of the television takes the dominant effect, then your eyes being less open may strain to see in the darkness of the room.
Remember the point about how the television screen is constantly changing the amount of light that it emits. As this happens, your pupils are constantly increasing and decreasing in size, which could be good exercise for your eyes or it could be a good explanation for why you are getting a headache or for why you are falling asleep.
I can think of two ways to offset these concerns. One is to have a light, other than the television, turned on in the room when you are watching television. The other is to not watch television.