The notion that staring at a screen for long periods of time is bad for POSTURE is true if you don't pay attention to how you sit/lay/stand while using it and position your neck relative to the screen but the added notion that it harms your eyes and optical health is only true if you don't do the following:
There is something called 'blue light' that is used in all screens (TV to computer to phone) to make it appear 'fresh' and 'vivid' to you. This is very good in short bursts at making things very powerful and appear 'wow this screen is so good' but is extremely detrimental to your eyes after long periods of use and somewhat ironically makes your eyes quite 'unblue' and red after long periods of staring at it. This even can damage your ability to process light if you 'toughen yourself up' to it and destroy your retina by making yourself seemingly immune to the eye strain's felt pain and irritating via conditioning yourself to it by extensive exposure. This is NOT a conspiracy theory, many screen-based technology providers acorss multiple platforms have realised this is the single thing harming people like me who are staring at the screen all day but for whatever reason failed to advertise or make 'big' their in-built solution in all models of things made post-2014 towards October 2015. They added something that Google calls 'amber lighting' but most call 'night light' (and Google ended up using that term too). It artificially makes all colours that stray away from orange to be toned down in what is shown to you (they can't actually 'cut out' or 'reduce' the 'blue light' element to the screen). What this does in turn is make your eyes receive a less vivid but extremely soothing version of whatever you are seeing whereby the glare and harshness is completely blunted if you set the strength at the right level but don't go all the way 'red' or 'extreme amber' nor forget the role that BRIGHTNESS plays in the harm to your eyes.
A key aspect to most tech with screens (if not all by this day and year) providing amber-increasing screen intonation is that it enables you to not have to reduce the brightness to such a severely dark level that your eyes are strained ANYWAY as you have to make the screen so dark that you struggle to 'focus' on it relative to the brightness of the lighting around you. This means you can now have your screen as bright (although i suggest 'one notch darker') than the lighting of your room/environment but have it ambered to such an extent that the focusing on the screen doesn't 'hurt' or redden your eyes for a long time.
Note: if you still experience eye-strain after actively using this, you need glasses and have abnormal eye-sight-range, don't be ashamed to see an optician, I guarantee you that if you have eye-strain after using amber lighting in the way I suggested, you 100% need glasses/contacts/laser-surgery so it won't be a wasted visit to the opticians.
Some platforms, such as Windows, 'force' you to set a time that you consider 'night' for the amber lighting to be activated during and it will turn back to blue-light-inclusive non-ambered screen for the rest of the time. A trick to get around this is to leave a fifteen-minute gap where your 'end time' is fifteen minutes earlier than the time of the 'start'. If you set it so they are the same time, it will never activate and you can't be more precise than 15-minute-slots as that's how Microsoft has decided to do it for now. When the fifteen-minute period begins (or when you are in a period of time where you know you'll be on during the fifteen minute period) set is for a fifteen minute period far away form the time you're using it. Do this constantly, there's no other solution. You want to be constantly doing it and never staring at non-ambered screen, this does what 'gaming glasses' do without costing a single extra cent or penny beyond what you paid for your system.