Brief Answer:
Several possibilities.

Detailed Answer:
I read your question carefully and I understand your concern.

There can be several causes for morning headaches. One simple cause may be incorrect posture during sleep which leads to muscle spasms. Perhaps changing the pillow to an anatomical one, in order to ensure that the curvature of the neck is similar to when staying erect might help.

Sleep issues like sleep apnea may be another cause, interruptions of breathing during sleep may lead to morning headaches and sleepiness and fatigue during the day. It should be particularly suspected if you are overweight and snore a lot at night.

Teeth grinding may be another common cause. It can lead to muscle spasms and headache upon awakening.

Sinusitis may be another cause which should be suspected if there is also nasal congestion and discharge.

Tension type headache, the most common type of primary headache (primary meaning with no identifiable cause), often triggered by stress and anxiety may also manifest in the morning in some patients. Usually alleviated by Ibuprofen. Migraine is another type of primary headache manifesting often in the morning but usually attacks last less than 72 hours so unlikely.

Last possibility would be the one of a brain lesion such as a tumor which can lead to headache more pronounced in the morning due to the increased intracranial pressure. It is unlikely as usually there are other signs like weakness or numbness of the limbs on one side, visual issues, balance and coordination difficulties, changes in behavior, seizures etc. However if headache persists over the next few weeks with no obvious cause, head imaging (by CT or even better MRI) even in the absence of other signs is advised to rule out brain lesions.

I remain at your disposal for other questions.

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