Sleep disturbances are sometimes a trigger for people who suffer from migraine. Sleeping more or less than you usually do can qualify as “disturbance.” See a doctor if you get such headaches regularly.

i’ve had a headache right after waking up for about a week. the previous week i had been staying up all night about every other night (final exams). just today after i woke up early and then took about a 5 hour nap later, i dont have a headache anymore.

Dehydration.

Teeth grinding usually occurs during your deepest sleep, which can occur just prior to waking, if you’re rest deprived or over-tired. That action can cause the headache. But, I think a lot of the time it’s just grogginess of the mind trying to pull out of the deep snooze and accept the pull of gravity and extra oxygen flow.