Bright light therapy is an established treatment pathway for sleep and circadian disorders and evidence suggests that it has antidepressant effects. The underlying mechanisms of these antidepressant effects are not fully understood and results from previous studies are somewhat variable. One of the important limitations of previous depression studies has been the heterogeneity of samples in which bright light therapy has been administered.

The main aim of this study is to evaluate whether the antidepressant effects of phototherapy in young persons with depression are modulated by changes in the sleep-wake cycle. We hypothesize that more pronounce initial sleep-phase delay will predict better antidepressant response to phototherapy and that the magnitude of changes in depressive symptoms across the course of the intervention will correlate with changes in the sleep-wake cycle.

Inclusion Criteria:

1. Quick Inventory of Depressive Symptomatology score > 6;
2. First episode of depression before age 25;
3. Currently engaged in, or about to start treatment for an affective disorder at a mental health clinic.

Exclusion Criteria:

1. Evidence of other sleep, neurological or primary medical conditions that could explain the current depression and/or contribute to sleep-wake dysfunction;
2. Other primary psychiatric disorders aside from anxiety disorders;
3. Significant alcohol or other substance dependence;
4. Use of medications that affect sleep, circadian rhythms, or alertness within the past month (participants stabilized on an antidepressant medication, stimulants, lithium or melatoninergic agents will not be excluded from the study);
5. Use of medications that may interact with light to produce a photoallergic reaction;
6. Eye or skin condition which may interact with bright light exposure;
7. Regular shift-work within 60-days prior to entry into the study;
8. Recent transmeridian travel.