yes, you mention drinking water - dehydration can lead to dizziness and headaches. I assume you've had blood tests, urine tests and other tests, and all came back normal?

I mean the other related factors that can cause lightheadedness is -sensitivity to light (moreso if you suffer migraines though) are you squinting your eyes all the time,
-eye strain by looking at computer screens or TV's for long periods of time for example
-Insomnia, which has a whole load of things such as no sleep, waking at early hours, odd sleeping positiion (such as putting strain on your neck if your bed/pillows/matress are old or broken),
-any medication you're on which brings on lightheadedness/drowsiness as a side effect.

But these are just suggestions, not something to worry about and scrutinise

When i went to A&E (emergency room for the americans amongst us!) with my first ever head zap & migraine the doctor did a few simple tests to show that the brain is functioning properly and its nothing to worry about. The jist of it is, hand/eye/leg/ movements & coordination, which if you pass all his tests, proves there's nothing wrong because if there was something seriously wrong with the brain, you wouldn't be able to do the tests! Simple really!