A patient wakes with a severe headache that lessens during the day. What is the likely diagnosis?

The scenario describes a headache that is severe upon awakening and tends to diminish as the day progresses. This pattern strongly aligns with cluster headaches, which are often characterized by bouts of intense pain that can be cyclical and may ease over time throughout the day.

Cluster headaches typically occur in groups or "clusters," and their occurrence can be associated with specific times, such as during sleep or early in the morning. This aligns with the patient's experience of waking up with a severe headache. The nature of the pain is also typically very intense, often described as sharp or burning, and can be accompanied by other symptoms such as nasal congestion or tearing on the same side of the head.

In contrast, migraine with aura often presents with other neurological symptoms and does not typically resolve over the course of the day as described. Increased intracranial pressure usually leads to headaches that can be persistent and worsen with certain positions or activities rather than improving as the day progresses. The option mentioning corticosteroid nasal spray does not refer to a diagnosis, and thus is not applicable in this context.

Overall, the description of the headache aligns best with characteristics of cluster headaches, making it the most likely diagnosis.