Dr. El-Ad of the Sleep Medicine Center, Technion - Israel Institute of Technology explains the pathophysiology and treatment of this circadian rhythm sleep disorder. Seen mainly in individuals with neurodegenerative disorders causing dementia, irregular sleep-wake pattern may further complicate the care of these patients. Institutionalization may follow the emergence of the irregular sleep-wake pattern because it can make a caregiver’s already difficult job unbearable. A rich environment and sufficient timely exposure to bright light may minimize the disturbance. Medical therapy with melatonin was generally ineffective in randomized control trials, despite encouraging open studies in small series of patients. The review enables basic understanding of the circadian clock biology and its interactions with the sleep-wake cycle.