Spontaneous Intracranial Hypotension (SIH): This condition is characterized by a decrease in cerebrospinal fluid (CSF) pressure, often due to a spontaneous leak. The positional headache that improves when lying down is a hallmark symptom, as standing exacerbates the decrease in CSF pressure, leading to headache. Night sweats can occur due to the body's response to the decreased intracranial pressure.

Migraine: Although migraines are typically not positional, some variants can present with postural exacerbation. Night sweats can be associated with migraine attacks in some individuals.
Tension-Type Headache: While less likely to have a clear positional component, tension-type headaches can occasionally worsen with standing due to increased muscle tension. Night sweats can be a non-specific symptom associated with various conditions, including tension headaches.
Anxiety or Stress-Related Headaches: Anxiety and stress can lead to both headaches and night sweats. The positional component might be less pronounced but could be influenced by changes in posture affecting breathing and muscle tension.

Subarachnoid Hemorrhage (SAH): Although SAH typically presents with a sudden, severe headache (often described as "the worst headache of my life"), it's crucial not to miss this diagnosis. Night sweats could be part of the body's response to the hemorrhage. The headache might not always have a clear positional component, but any sudden onset of severe headache warrants immediate investigation.
Meningitis: Meningitis can cause headache and night sweats. While the headache in meningitis is typically not positional, the severity of the condition necessitates its inclusion in the differential diagnosis to ensure prompt treatment.
Pituitary Apoplexy: This condition involves sudden hemorrhage or infarction of the pituitary gland and can present with severe headache, which might have a positional component. Night sweats could be part of the endocrine dysfunction.

Cerebral Venous Sinus Thrombosis: This condition involves a blood clot in the brain's venous sinuses and can cause headache. While the headache might not typically be positional, the condition's rarity and potential severity warrant consideration.
Chiari Malformation: Some types of Chiari malformation can cause headaches that worsen with standing due to the displacement of cerebellar tonsils into the spinal canal. Night sweats could be associated with the autonomic dysfunction sometimes seen in Chiari malformations.
Idiopathic Intracranial Hypertension (IIH): Although IIH typically presents with headache that worsens with standing due to increased intracranial pressure, it's less likely given the improvement with lying down. However, it remains a consideration in the differential diagnosis due to its potential for serious complications if left untreated.