Blue light can lift seasonal depression, but the dose–response is unclear
In seasonal affective disorder, depressive symptoms tend to recur in winter, and light therapy is used to mimic a longer summer photoperiod [4]. One study found that narrow-band blue light at 468 nm significantly reduced SAD symptoms [5]. That wavelength falls in the blue part of the visible spectrum, and the finding suggests that blue light itself can have an antidepressant effect.
What is not established is how that benefit changes as blue light intensity rises. A review of bright light therapy trials compared outcomes in people exposed to different light conditions, including low-intensity blue light, but the available summary does not describe a clear intensity–benefit curve [2]. So the practical answer is that blue light can help, but the evidence does not yet show whether more intense blue light produces proportionally more relief or whether the benefit plateaus.
Headaches are a known side effect of bright light, but their link to blue light intensity is not measured
Prolonged exposure to bright light can cause eye irritation, headaches, and nausea [1]. That is a general warning about bright light therapy, not a finding specific to blue light intensity. A review of bright light therapy for SAD also lists headaches among side effects, alongside insomnia and light-related complaints [2].
The gap is that none of the available studies reports how headache frequency or severity changes as blue light intensity increases. The headset study examines antidepressant effects and side effects such as headaches, but its abstract does not provide a dose–response analysis for blue light [1]. So a reader should not assume that raising blue light intensity will predictably increase headaches; the direction and size of that change are not documented in these sources.
Light therapy is an alternative to antidepressants, but the side-effect trade-off is different
Light therapy is often compared with antidepressant drugs for SAD. One comparison notes that antidepressants can cause palpitations, dyspnea, dizziness, headache, and weight loss, and that newer studies have been investigating blue light therapy [3]. That makes the balance question practical: a person may weigh the headache risk of light therapy against the broader side-effect profile of medication.
However, the same comparison does not analyze headaches from light therapy or how they change with blue light intensity [3]. A review of SAD treatments focuses on efficacy of light therapy and medications without addressing headaches or the benefit–side-effect balance as blue light intensity changes [4]. So the evidence supports light therapy as an effective option, but it does not give a clear rule for tuning blue light intensity to maximize benefit while minimizing headaches.
Sources used in this answer
Light therapy headset model for seasonal affective disorder group
A light therapy headset model for seasonal affective disorder examines antidepressant effects and side effects such as headaches, and notes that prolonged bright light can cause eye irritation, headaches, and nausea.
Is bright light therapy effective in treating depression in adults with seasonal affective disorder?
A review of bright light therapy for seasonal affective disorder assesses efficacy on depressive symptoms and lists headaches among side effects, but does not report how headaches change with blue light intensity.
Comparing Light Therapy and Fluoxetine Therapy in Seasonal Affective Disorder
A comparison of light therapy and fluoxetine for seasonal affective disorder discusses efficacy and antidepressant side effects including headache, but does not analyze light-therapy headaches or blue light intensity.
Treatment of seasonal affective disorder: a review
A review of seasonal affective disorder treatments evaluates light therapy and medications for efficacy but does not address headaches or the balance between benefits and side effects as blue light intensity changes.
Seasonal affective disorder and light therapy
A study of seasonal affective disorder and light therapy reports that narrow-band blue light at 468 nm significantly reduced SAD symptoms, without providing information on side effects such as headaches.