Hello and welcome to HCM.
Thank you for your query.
Long duration of mobile phone usage has reported to cause a headache in majority of mobile phone users. 5 hours is a lot and this could be the cause. I would advise you to first either reduce the duration or split it, with breaks in between; if this is possible.
The next thing, a headache due to most of these conditions could have varying causes. The first thing to do would be to visit an ophthalmologist for a complete eye examination. The next thing to do would be to practice good techniques like: sitting at a distance from the television set, using the computer for short duration's, giving your eyes rest, etc.
As you have spoken about a headache present only on one side, this is typically seen in migraine headaches
. You will have to consult a doctor for this, once all other conditions are ruled out. This can be avoided with a healthy lifestyle and knowledge on the condition.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications. I would gladly help you.