We describe a 63-year-old man diagnosed with sporadic Creutzfeldt-Jakob disease (sCJD), specifically sporadic fatal insomnia, confirmed through real-time quaking-induced conversion (RT-QuIC) analysis of cerebrospinal fluid and polysomnography. He presented with rapid cognitive decline, behavioural changes, sleep disturbances and dysautonomic symptoms. Initial MR imaging, electroencephalogram and cerebrospinal fluid analyses were inconclusive, highlighting the difficulty in diagnosing this rare subtype of CJD. Clinical evaluation is fundamental in defining the diagnosis of sCJD. When clinical suspicion is strong, the diagnostic work-up should be continued. In this case, the combination of comprehensive clinical evaluations and advanced diagnostic tools, including RT-QuIC and polysomnography, proved essential in making a definitive diagnosis.