It is never a good idea to watch videos or play games on your phone in the dark without much light. Your eyes would be straining to focus on the screen and over time, this stress would add up and cause your eyesight to go bad. In the dark, the blue light emitted by TV or Mobile phones are more intense and it could hurt your eyes in the long term.
I know some people love doing just that and they somehow get away with not needing to wear any glasses in life. They are the exceptions and perhaps their genetics are in play here. Some people are born with weak eyesight, even if they don't spend much time in front of a TV or Mobile phone screen. Of course, frequently doing such activities as watching extreme videos on phone or TV is one of the primary reason for bad eyesight, there are other contributing factor as well.
It is a proven fact, that if you use enough light to watch videos or TV, your eyesight wouldn't go bad or if it does, it would be a slow process. I find that modern days, LED lights are great for rooms throughout the house. Ever since I started using LED lights with 5k (daylight brightness) all over my house lightning fixtures, my eyesight had actually remained the same for many years.
Even my eye doctor was surprised that my eyes hadn't gone bad in many years. I do wear low power eye glasses and it had stay the same for years consecutively. You also need to practise "Dr Bates eye exercises" to maintain the strength in your eye muscle. You will be surprised that such eye exercises, really helps in improving or at least maintaining your eyesight. Search Google for Dr Bates eye exercises. Some of the information you can get for free while others you need to purchase an ebooks.
Eating habits is also one of the major factor that will determine the health of your eyes. Eat enough carrots, eggs, green leafy vegetables, nuts, and drink plenty of water. After all, your eyes needs enough liquid to maintain the shape of your eyeballs.
Practise rubbing the sides and corners of your eyes gently after a tiring day, this brings fresh blood into your eyes and gets the blood flowing smoothly through your eyeballs and surrounding area.
Have you ever tried closing your eyes very firmly and then opening your eyes quickly? You will temporarily feel like as if you can see so much better. That's because when you close your eyes firmly, and then open your eyes, it causes your blood to flow quickly into your eyeballs. With enough blood in your eyeballs, it maintains the perfect shape which allows your eyes to see better. But as your day goes by, and you use your eyes to see items at a close distance like watching TV or Mobile Phone, your eye balls changes back to its original imperfect shape. Then your eyesight will start to feel blurry and you can't see properly without glasses.