Tension Headache: This is often the most common type of headache and can be exacerbated by lying down due to increased pressure on the neck and scalp muscles.

Sinusitis: Sinus pressure can worsen when lying down, leading to headaches. This is due to the accumulation of mucus and increased pressure in the sinuses.
Migraine: While migraines can occur at any time, certain positions, including lying down, can trigger or worsen migraine headaches in some individuals.
Cervicogenic Headache: These headaches are caused by referred pain from the cervical spine and can be exacerbated by certain positions, including lying down, especially if there is an underlying neck issue.

Subarachnoid Hemorrhage: Although rare, a sudden, severe headache that worsens when lying down could be indicative of a subarachnoid hemorrhage, which is a medical emergency.
Meningitis: Headache when lying down, especially if accompanied by fever, stiff neck, and confusion, could indicate meningitis, an infection of the protective membranes covering the brain and spinal cord.
Increased Intracranial Pressure (ICP): Conditions that increase ICP, such as brain tumors or hydrocephalus, can cause headaches that worsen when lying down due to increased venous pressure.

Chiari Malformation: A structural defect in the cerebellum that controls balance can cause headaches, especially when lying down, due to the increased pressure on the brainstem.
Arteriovenous Malformation (AVM): A rare condition where there is an abnormal connection between arteries and veins, which can cause headaches, including those that worsen when lying down, due to the altered blood flow dynamics.
Idiopathic Intracranial Hypertension (IIH): Also known as pseudotumor cerebri, this condition involves increased pressure within the skull without a detectable cause, leading to headaches that can worsen when lying down.