Does staring at an ipod touch screen for a long time cause eye power loss?

I already wear glasses. And I am afraid that things such as being on my ipod, tv or computer for a long time will cause an eye power loss. Can someone please tell me ways that make eyes lose their power, and ways to prevent it? For example how long should people be looking at electronic screens? Thankyouu.|||it should be the standard times, like take a 15minute rest for every 2hours. i wouldn't think it would make a huge difference than watching tv, typing etc,|||You can't hurt your eyes by using them...The real problem with your activities is loss of brain power|||It is not recommended to look at screens for long periods of time.