The goal of this observational study is to characterize the pupil light response in patients with delayed sleep-wake phase disorder. A subset of participants will take part in a clinical trial to determine if increasing serotonin can alter the circadian response to light. The main questions it aims to answer is:

Are there differences in the light response in patients with delayed sleep-wake phase disorder? Does citalopram increase melatonin suppression in patients with delayed sleep-wake phase disorder?

Participants will complete activity and melatonin testing at home to determine their circadian timing. This will be followed by an eye test looking at the pupil response to different light stimuli. A subset of participants will complete two inpatient admissions where melatonin levels will be sampled and they will be exposed to a bright light. During one round of testing they will receive a placebo and in the other round of testing they will take a single dose of citalopram, a selective serotonin reuptake inhibitor.

Inclusion Criteria:

• Delayed sleep-wake phase disorder

Exclusion Criteria:

• Use of medications known to alter pupil diameter
• History of eye disease or eye surgery
• Unstable medical or psychiatric condition
• Shift work or self imposed irregular sleep schedules
• Caffeine consumption >400mg/day