Tired eyes and font size

Here's an advantage to ereaders over paper which has not often been mentioned:
Not only can you set a large font if your eyes are old or poor, but you can also do so simply if you're tired. It's amazing how an increase in text size eases off the eye strain, even if one is simply tired at the end of a long day.

Also, doing so before one gets tired or the eyes do, may increase the length of time one can read without discomfort or risk of damage. Space is not limited on an ereader, set the font as large as you want!