Circadian and seasonal rhythm disturbances are prominent in patients with psychiatric disorders. Properly timed and dosed light of specific spectral composition stabilises mood and sleep through serotonergic mechanisms and through input to the master circadian clock in the hypothalamus. Correctly administered, light can be used as an effective treatment for seasonal and non-seasonal depression and for stabilising the sleep-wake cycle. Blocking blue light in the evening may provide a non-pharmacological anti-manic tool. Current developments use dynamic lighting built into somatic and psychiatric hospitals to maximise the beneficial effects of light.