- Chronic SWSD-Diagnosis made on the basis of at least minimum ICSD criteria. Furthermore, patients with chronic SWSD must have had excessive sleepiness during night shifts for at least 3 months, work a minimum of 3 night shifts per month that include at least 6 hours between 2200 and 0800 and are no longer than 12 hours in duration, and plan to continue to work night shifts throughout the study.