[trux]

Actually, the headache at freediving is usually caused by CO2 excess, not really by hypoxia. CO2 in high levels is toxic, and headache is one of the symptoms of it. That told, it does not mean you are reaching any CO2 levels dangerous to your life or health. You will probably get used to it sooner or later, to certain degree. You can also try making longer recoveries between breath-holds - ventilate well after the performance to evacuate CO2 and other waste products, but avoid hyperventilating before the next one! Good but not excessive hydratation as pointed out above by others, is important of course too.