You wake up with a headache because of the excess blood flow to your brain while you sleep.
As you fall asleep, your body begins to relax and go into a state of unconsciousness. As this happens, blood flow slows down and is redirected to other parts of the body. The brain is so important that it receives a large amount of blood flow during this time, which can cause headaches in some people. If you wake up with a headache, it’s because there was too much blood flowing to your brain while you slept.
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