Seasonal Affective Disorder (SAD) is a subtype of Major Depressive Disorder, characterized by a recurrent temporal relationship between the season of year, the onset and the remission of a major depressive episode. Estimates of the annual prevalence state that 1-6% of the population will develop SAD with the larger prevalences found at greater extremes in latitude. SAD is most likely triggered by the shortening photoperiod experienced in the winter months leading to a deterioration of mood. Recent cross-sectional neuroimaging studies have found cellular and neurotransmitter changes in response to seasonality, ultimately having an impact on the affect of patients. Conversly, this study aims to investigate the changes in neurocircuitry related to depression and euthymic states. Patients with SAD offer a unique ability to study these changes since they have predictable triggers for the onset of depression (i.e. the winter months) and remission (i.e. the summer months).

Inclusion Criteria for SAD Cohort

1. Male or female between the ages of 18 to 65 years, inclusive
2. Patients who are able and willing to give consent and able to attend study visits
3. Agreement to use light therapy for four weeks
4. DSM-V diagnosis of seasonal affective disorder, at least 2 year history of the illness with a Structured Interview Guide for the Hamilton Depression Rating Scale-SAD version (SIGH-SAD) score ≥ 25 at screening

Exclusion Criteria for SAD Cohort

1. Current alcohol and/or substance use disorder
2. Use of cigarettes
3. Past or present psychiatric disorders (axis I and II) other than SAD
4. Taken medications approved and/or employed off-label for depression
5. Previous use of light therapy
6. Use of photosensitive medications
7. Montreal Cognitive Assessment score < 24
8. Patients with standard contraindications for MR imaging. For example, non-MRI compatible metallic implants including cardiac pacemaker, size limitations etc.
9. Known intolerance or allergies to MRI contrast agent (Gadolinium or Magnevist) including advanced kidney disease
10. Severely impaired renal function (estimated glomerular filtration rate <30ml/min/1.73m2)
11. Individuals who are not able or willing to tolerate the required prolonged stationary supine position during treatment
12. Pregnant and/or breastfeeding
13. Travelled to another a more southern latitude within 6 months of scan
14. Night shift workers
15. Are participating or have participated in clinical trial or research study in the last 30 days
16. Unable to communicate with investigator and/or staff
17. Diagnosis of a reading disability, dyslexia or significant learning disorder

Inclusion Criteria for Unipolar Depression Cohort contraindications

1. Male or female between the ages of 18 to 65 years, inclusive
2. Patients who are able and willing to give consent and able to attend study visits
3. DSM-V diagnosis of major depressive disorder, with a Hamilton Depression Rating Scale score ≥ 22 at screening

Exclusion Criteria for Unipolar Depression Cohort

1. Current alcohol and/or substance use disorder
2. Past or present psychiatric disorders (axis I and II) other than SAD
3. Montreal Cognitive Assessment score < 24
4. Patients with standard contraindications for MR imaging. For example, non-MRI compatible metallic implants including cardiac pacemaker, size limitations etc.
5. Known intolerance or allergies to MRI contrast agent (Gadolinium or Magnevist) including advanced kidney disease
6. Severely impaired renal function (estimated glomerular filtration rate <30ml/min/1.73m2)
7. Individuals who are not able or willing to tolerate the required prolonged stationary supine position during treatment
8. Pregnant and/or breastfeeding
9. Are participating or have participated in clinical trial or research study in the last 30 days
10. Unable to communicate with investigator and/or staff
11. Diagnosis of a reading disability, dyslexia or significant learning disorder

Inclusion Criteria for Healthy Controls

1. Male or female between the ages of 18 to 65 years, inclusive
2. Patients who are able and willing to give consent and able to attend study visits
3. No current or past history of mental disorder
4. No unstable medical disorders

Exclusion Criteria for Healthy Controls

1. Use of any medication for a general medical disorder and/or condition that, in the opinion of the investigator, may affect neural structure
2. Alcohol or drug-use within 24 hours of MRI
3. Pregnant and/or breastfeeding
4. Montreal Cognitive Assessment score < 24
5. Patients with standard contraindications for MR imaging. For example, non-MRI compatible metallic implants including cardiac pacemaker, size limitations etc.
6. Known intolerance or allergies to MRI contrast agent (Gadolinium or Magnevist)
7. Individuals who are not able or willing to tolerate the required prolonged stationary supine position during treatment
8. Are participating or have participated in clinical trial or research study in the last 30 days
9. Unable to communicate with investigator and/or staff