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Light therapy matters because seasonal affective disorder (SAD, a pattern of depression that recurs each winter) is thought to arise from reduced light exposure disrupting the body's internal circadian (daily) rhythm. Bright light exposure each morning, typically around 10,000 lux (a measure of light intensity, far brighter than ordinary indoor lighting), can correct this disrupted rhythm and relieve the winter depressive episode. For example, imagine a patient whose low mood, oversleeping, and carbohydrate craving return every autumn and lift every spring: this seasonal pattern is the clue that points toward SAD and light therapy as a first-line option. Because the mechanism is circadian rather than convulsive or surgical, light therapy carries none of the anaesthetic, seizure, or surgical risks of the other physical treatments.

Light therapy works by resetting the circadian (daily) rhythm and suppressing melatonin (a hormone that signals darkness and promotes sleep), which is thought to be dysregulated in seasonal affective disorder (SAD). A light box delivering around 10,000 lux is used each morning, usually for twenty to thirty minutes, because morning light has the strongest effect on resetting the body clock. Say a patient instead uses the light box in the evening: the antidepressant effect is far less reliable, because timing relative to the sleep-wake cycle matters as much as the light intensity itself. In practice, side effects are mild, headache and eye strain, but caution is needed with photosensitising drugs (drugs that increase skin or eye sensitivityThe proportion of people who truly have a condition that a test correctly identifies as positive.Read more → to light), which can interact with bright light exposure.

• What intensity and duration of light are conventionally used for seasonal affective disorder?
• Through which retinal cells does bright light reset the circadian clock?
• Which risk should be discussed before starting light therapy in someone with bipolar disorder?

• Electroconvulsive therapy (ECT), requires anaesthesia and induces a seizure, unlike light therapy which is entirely non-invasive with no seizure risk.
• Transcranial magnetic stimulation (TMS), stimulates the brain directly through magnetic pulses, whereas light therapy acts through the eyes on the circadian (daily) rhythm.
• Major depressive disorder without a seasonal pattern: treated with standard antidepressants or psychological therapy, unlike SAD, where morning light therapy is a first-line option.
• Photosensitising drugs (drugs that increase skin or eye sensitivityThe proportion of people who truly have a condition that a test correctly identifies as positive.Read more → to light): need caution alongside light therapy, unlike the brain-stimulation treatments, which carry no light-related interaction.

• Very dim light at night would not deliver the therapeutic signal.
• This intensity approaches direct sunlight and is not used therapeutically.
• Ordinary indoor lighting of this intensity is insufficient.
• Both the intensity and the duration are too low to be effective.
• Cones serve colour and detail vision.

• Identify the drug, receptor, enzyme or pharmacokinetic change named in the stem.
• Translate it into the immediate mechanism before looking at the options.
• Use timing and the adverse-effect pattern to separate the nearest alternatives.
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