LPT: If you suddenly start waking up every morning with a headache, check to see if your bed is level.

If your mattress has sunk at one end, or something else has caused your head to be even a little bit lower than the rest of your body, you are very likely to wake up with a headache until you fix that.

EDIT: This is merely a suggestion of one of many possible explanations. Many people have mentioned sleep apnea, carbon monoxide, etc. Those are all valid points. This was what happened to me, so I wanted to share, because it’s not an obvious thing.