Delirium: This is the most likely diagnosis due to the progressive nature of confusion throughout the day. Delirium is characterized by an acute onset of fluctuating cognitive impairment, often with altered levels of consciousness, which can worsen over the course of the day.

Dementia: While dementia typically presents with a more gradual decline in cognitive function, certain types (like vascular dementia) can have a step-wise progression that might seem like a daily worsening of symptoms.
Medication Side Effects: Many medications, especially those with anticholinergic properties, can cause confusion as a side effect. The timing of medication intake could correlate with the progression of confusion throughout the day.
Sleep Deprivation: Lack of sleep or disrupted sleep patterns can significantly impair cognitive function, leading to confusion that worsens as the day progresses due to accumulating fatigue.
Hypoglycemia: Low blood sugar can cause confusion, and if not properly managed, could lead to worsening symptoms throughout the day, especially in diabetic patients.

Stroke or Transient Ischemic Attack (TIA): Although less common, a stroke or TIA can present with sudden confusion. The progression could be due to evolving neurological deficits.
Meningitis or Encephalitis: Infections of the brain or meninges can cause severe and progressive confusion, along with other symptoms like fever, headache, and stiff neck.
Septicemia: Systemic infection can lead to confusion (septic encephalopathy) that worsens as the condition progresses, especially if not promptly treated.
Hyperglycemic Hyperosmolar Nonketotic Syndrome (HHNS): A serious complication of diabetes that can cause confusion among other symptoms, which can worsen without treatment.

Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause rapidly progressive dementia and confusion.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by cognitive impairment and confusion, which can fluctuate.
Wernicke's Encephalopathy: A neurological disorder caused by thiamine deficiency, often seen in alcoholics, which can present with confusion, ataxia, and ophthalmoplegia.
Subdural Hematoma: Especially in the elderly, even minor trauma can lead to a subdural hematoma, which can cause progressive confusion among other neurological deficits.