Major depression may be associated with abnormal circadian rhythms (biological clocks). For example, patients reach tachycardia (dreams occur during the rapid eye movement phase) more quickly and more intensely. Tachycardia must be reached when 5-hydroxytryptamine levels in the brainstem are reduced and is affected by compounds that increase 5-hydroxytryptamine levels in the brainstem (e.g. antidepressants). Overall, the 5-hydroxytryptamine system is least active during sleep and z most active during wakefulness. Prolonged wakefulness due to sleep deprivation activates 5-hydroxytryptaminergic neurons, causing similar effects to those of antidepressants such as selective 5-hydroxytryptamine reuptake inhibitors. After a night of sleep deprivation patients may feel significantly lighter in mood. Selective 5-hydroxytryptamine reuptake inhibitors may rely on an increase in central 5-hydroxytryptaminergic transmission to achieve their efficacy, a system also associated with the sleep-wake cycle.
Studies on the efficacy of light therapy for seasonal affective disorders suggest that light deprivation is associated with reduced activity of the 5-hydroxytryptaminergic system and abnormal sleep cycles (particularly insomnia). Light exposure enhances the action of the 5-hydroxytryptamine system, a system that may play an important role in depression. Sleep deprivation and light therapy as well as antidepressants both target the same brain-derived neurotrophic factors and brain regions. These therapies are now used in clinical treatment. Light therapy, sleep deprivation and sleep time-shifting (sleep cycle advancement therapy) have been used in combination to rapidly break down deep depression in hospitalised patients.
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