Brief Answer:
Several possible causes.

Detailed Answer:
I read your question carefully and I am sorry about the symptoms you are experiencing.

The causes of headache can be so many, the description you make is not that specific, could correspond to many possibilities. Among these there is also dehydration. So in theory it could be a possible explanation. However it is something which you should be able to correct by increasing your intake of water, so I wouldn't really see it as a cause of what seems like a chronic headache. Also would be good to know some more info, such as time of onset and evolution of symptoms in time, type of pain (throbbing, pressure-like etc), other phenomena (nausea, vomiting, red eye, nasal congestion and discharge, fever etc), duration of headache attacks, other medical conditions (like high blood pressure) etc

I would perhaps consider some other cause before dehydration. One common cause might be tension type headache. It is the most common primary headache in the population (primary meaning no identifiable cause), often triggered by stress and anxiety. Apart from Ibuprofen for the pain, measures to relieve stress like relaxation techniques, light exercising, sports like yoga may help. There is also preventive medication like amitriptyline, but I would try to avoid that, only if sure of the diagnosis and the rest doesn't work.

Another very common cause of persisting headache especially around the eyes might also be sinusitis, even more probable if you have nasal congestion. For that a physical exam, perhaps imaging of the sinuses are necessary and if confirmed nasal decongestants, at times antibiotics are necessary.

Other causes may be eyesight issues, if your vision is not could there can muscle tension from straining.

Brain lesions are not likely in the absence of other symptoms. If there are other symptoms you forgot to mention, or appear anew I remain at your disposal to discuss them.

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Brief Answer:
Read below.

Detailed Answer:
Thank you for the additional info.

Judging by that information, yes, sinusitis is not that likely.
I didn't explain myself well I am afraid by asking about onset, I am sorry. While the time of day is useful as well, I had in mind since when have these headaches appeared, whether weeks, months, years. Have they been present in the past as well, becoming only more frequent and bothering now, or is it a new phenomena which is progressing in intensity.

I would say the previously mentioned tension type headache remains a possibility. More probable if the headaches seem to be related to stress, anxiety, work overload. The above recommendations remain the same.

If you do not feel the headache is not influenced by the above factors and the suggested measures are ineffective, then brain imaging should be scheduled. As I said in the absence of other symptoms brain conditions are unlikely so I wouldn't want to alarm you by saying that, but in an unexplained persistent headache it is advisable, especially if it has appeared only in recent months.

I hope to have been of help.

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