I only ever get headaches when I go to bed late.
I have mild insomnia so I end up going to sleep around 3am on average and even though I only have 8/9 hours sleep, so 11am, I still wake up with a headache. I drink through the night so I don't think it's dehydration, however I don't always get headaches when I go to bed late, but 90% that is when I get headaches.
I hardly get them through the day, I always wake up with one, I never get one at like 5 in the afternoon.
What could it be?