The goal of this clinical trial is to learn how the pupil responds to different light stimuli and how that relates to sleep and daily rhythms in healthy people of different ages.

The main questions it aims to answer are:

• Does the eye's pupil response to light stimuli differ by the sex and age of the participant?
• Is the eye's pupil response to light stimuli related to each participant's sleep timing, their body clock timing, and their hormone responses to light.

Participants will have a special eye exam and complete questionnaires before starting the study to see if they can participate. If they can participate, they will wear a special watch that monitors their activity and light levels for one week. Then they will live in a research room at the Mass General Hospital for 3 days/2 nights during which we will test their pupil response to light, their body clock timing, and their hormone responses to light.

Inclusion Criteria:

• (i) 18-85 years old.
• (ii) Habitual sleep onset 10 pm- 1 am (healthy controls only);
• (iii) Habitual wake onset 5:30 am- 8:30 am (healthy controls only);
• (iv) vision correctable to 20/30;
• (v) stable medically.
• (vi) ability to speak, understand, and read English at a high school level

Exclusion Criteria:

• (i) Color blind by Ishihara Color Blindness Test;
• (ii) any history of eye trauma, surgery or abnormality (e.g., retinopathy, glaucoma, cataracts, amblyopia, macular degeneration, congenital color vision deficiencies, or any type of blindness) besides correctable vision abnormalities (e.g., with glasses); any abnormalities on clinical eye exam (e.g., neuritis, Neuromyelitis optica, treated or untreated glaucoma) such that the ophthalmologist recommends the participant not be studied; Limited cataracts (e.g., Lens Opacities Classification (LOCS) III grade <2) will be allowed and documented during the eye exam. Eye drops that affect pupil size or contractility (e.g. mydriatics, miotics); drops to treat glaucoma (e.g., pilocarpine, brimonidine, other drops like artificial tear drops, or anti-inflammatory drops would not be exclusionary)
• (iii) current or history of neurologic or psychiatric disease including autonomic function disorder or migraines; psychiatric disorder requiring medications in a first degree relative (healthy controls only); limited-duration counseling without prescription medications will not be exclusionary; (iv) current or history of circadian rhythm sleep-wake disorder (healthy controls only);
• (v) prescription or non-prescription drugs affecting the pupil (e.g., affecting autonomic function), sleep, melatonin (e.g., lithium, alpha- and beta-adrenergic antagonists), and/or circadian rhythms (e.g., beta blockers, non-steroidal anti-inflammatory drugs, tricyclics);
• (vi) Other disorders that can affect or may be affected by intrinsically photosensitive Retinal Ganglion Cell (ipRGC) function, including diabetes mellitus, multiple sclerosis, Parkinson's disease, seasonal affective disorder;
• (vii) shift- or night-work in past three months; history of night work in preceding 3 year period
• (viii) crossing more than 2 time zones in past three months;
• (ix) presence of depression as assessed by a Beck Depression Inventory (BDI) score >14.
• (x) pregnant or less than 6 weeks post-partum or breast-feeding

Data sharing will be conducted per National Institutes of Health (NIH) and MassGeneralBrigham (MGB) policies and regulations. Only de-identified data will be shared.