While PhotoReading you only want a soft focus. If you're having trouble refocusing afterwards check that you might be crossing your eyes, remember to relax you are using divergent gaze like parallel viewing.
Feeling tired afterwards would not be unusual. Have a drink of water get up stretch walk around for a bit. You might be surprised how awake you feel 10 to 20 minutes later. It's pretty normal and can happen from time to time.
Alex
