I have already told you that it's bad for your eyes when you’er reading on the bed
个人觉得应该这样表达。用一个状语从句,用because的话,感觉太中式英语了。
I have already told you not to read in bed because it's bad for your eyes.
或者这样写吧。。仅供参考。
it's bad to your eyes.表达常识用一般现在时即可。
I've already told you it's bad to your eyes read in bed.