This study investigates whether the antidepressant effects of bright light therapy (BLT) differ between the spring/summer season and the autumn/winter season in adults with depressive disorders. While BLT is an established treatment for seasonal affective disorder, increasing evidence suggests it may also be effective for non-seasonal depression. However, it remains unclear whether its effectiveness depends on seasonal variation in natural daylight exposure.

In this prospective study conducted in a routine clinical care setting, patients referred for depressive symptoms and a score of > 6 on the Quick Inventory of Depressive Symptomatology-Self-Report (QIDS-SR) receive BLT in a structured "Light Café" environment. Participants undergo daily morning light therapy sessions (30 minutes at 10,000 lux) for at least one week, with possible extension up to three weeks based on treatment response. A delayed-start design is used, where a subset of participants begins treatment two weeks later, allowing comparison with the natural course of depressive symptoms.

Depressive symptoms are assessed using the QIDS-SR at baseline, during treatment, and at follow-up (6 weeks and 3 months post-treatment). Sleep quality, chronotype, circadian timing, and side effects are also measured.

The primary objective is to compare the effectiveness of BLT across seasons. Secondary objectives include evaluating the persistence of treatment effects, assessing the change in sleep quality and day-to-day sleep-wake pattern, and exploring whether outcomes vary by chronotype, circadian phase, and patient characteristics.

This study aims to inform whether BLT should be considered a year-round treatment option for depressive disorders and to better understand factors influencing individual treatment response.

Inclusion Criteria:

• Adults aged 18 years or older
• Diagnosis of a depressive disorder (unipolar or bipolar), including both seasonal and non-seasonal depression
• Currently indicated for bright light therapy
• QIDS-SR score ≥ 6 at baseline
• Able to provide informed consent
• Sufficient proficiency in Dutch or English to complete questionnaires

Exclusion Criteria:

• Current (hypo)manic or mixed episode
• Current psychotic episode
• Severe suicidal ideation requiring immediate intervention
• Diagnosis of dementia
• QIDS-SR score ≤ 5 at baseline
• Age < 18 years
• Inability to provide informed consent
• Insufficient language proficiency to complete study questionnaires
• Initiation, dose change, or switch of antidepressant medication within 3 weeks before or during light therapy
• Long-term use of agomelatine

Relative Contraindications (require consultation with treating physician before participation):

• Pregnancy (first trimester)
• Diabetes or other systemic vascular diseases
• Eye conditions associated with increased light sensitivity
• Epilepsy

Medication-Related Considerations:

• Participants are advised not to start antibiotics during light therapy (existing courses should be completed prior to participation)
• Participants are advised to discontinue use of photosensitizing medications or topical agents during treatment
• Participants are asked to discontinue melatonin use during light therapy
• A wash-out period of 1 month is required after discontinuation of melatonin or short-term agomelatine use before starting light therapy

The study population consists of patients with depression who are referred for treatment of depressive symptoms within a mental health care setting (GGzE, Eindhoven, The Netherlands), where bright light therapy is offered in a structured "Light Café" environment as part of routine clinical care.