Signature Response to Light Therapy in Unipolar and Bipolar Major Depressive Episode (MDE)

Patients : Inclusion Criteria: 1. Adults between 18 and 65 years old 2. Patients with a diagnosis of MDE as part of a unipolar or bipolar disorder (DSM-5 TR criteria) 3. Patients with a clinician-rated MADRS score ≥ 20 indicating a moderate to severe depressive symptoms with a prescription of LT for MDE treatment 4. For patients with bipolar disorder type 1: With prophylactic mood stabilizer treatment at an effective dose (standardized and at a preventive dose for manic episode prevention) and stable for at least 4 weeks (background treatment for bipolar disorder): Lithium (Téralithe® LP 400 or 250) or Sodium Valproate (Dépakote® 500) or Atypical antipsychotics (also known as second-generation antipsychotics), defined as follows and dosed prior to inclusion in the month preceding: * For Lithium, controlled by serum lithium level, \>0.5 mEq/L for Téralithe 250 and \> 0.7 mEq/L for Téralithe LP 400 ; for Sodium Valproate, controlled by serum sodium divalproate level \>40mg/L * For antipsychotics, only the following molecules will be accepted at the indicated dosages, as they have marketing authorization in France as prophylactic mood stabilizers for manic episode prevention in bipolar disorder: Quétiapine (≥150-800mg), Aripiprazole (≥ 15-30mg), Olanzapine (≥ 10-20mg) 5. Patients for whom light therapy is prescribed 6. Patients able to understand French. 7. Patients able to sign informed consent. Exclusion Criteria: 1. Patients with a DSM-5 diagnosis of schizophrenia, suffering from paranoid or delusional disorders, any other psychotic features or other nonstabilized mental disorders 2. Patients with other antidepressant strategies than LT or mood stabiliser initiated in the month prior to inclusion 3. Patients with a therapeutic resistance of current MDE (lack of response to ≥2 antidepressants of 2 different classes at therapeutic doses for \>6 weeks) 4. Patients with a LT used in the last 1 month 5. Patient with ophtalmic pathologies (cataract, glaucoma, age-related macular degeneration, severe myopia \> 6D, etc.) or diseases affecting the retina (retinitis pigmentosa, diabetes, herpes, hereditary retinal disorders etc.). 6. Patient with photosensitive epilepsy 7. Patients with a C-SSRS score ≥ 4 8. For the MRI group, patient presenting one or several MRI contrindications 9. Shiftwork or self-imposed irregular sleep schedule within the last months 10. Travel acrosss 2 time-zones during the last month prior to participation 11. Pregnant or breastfeeding women 12. Subject under guardianship or deprived of liberty 13. Participation ton another interventional study Healthy volonteers : Inclusion criteria : 1. Adults between 18 and 65 2. Without any significant psychiatric history or current psychotropic medication 3. Able to understand French 4. Able to sign informed consent Exclusion criteria : 1. Contra-indications to MRI (claustrophobia, metallic or material implants) 2. History of severe head injury 3. Pregnancy and breastfeeding woman 4. Inability to understand information about the study 5. Persons deprived of their liberty by judicial or administrative decision 6. Adult subject under legal protection or unable to consent