Phototherapy

Light as a biological signal

Light is a highly significant signal from the external environment that influences many physiological processes. Besides enabling image vision through rods and cones in the retina, it is the most important synchronizer of the biological clock and also directly influences areas of the brain regulating mood, cognitive functions and sleep. This so-called non-image “vision” is mediated by a third type of photoreceptor in the retina, the intrinsically photosensitive retinal ganglion cells (ipRGCs).

Mechanism of phototherapy effect

The mechanism of the phototherapy effect is mediated by the ipRGC pathway. These cells form part of the optic nerve and their axons affect many areas of the brain. The dominant part leads to the hypothalamic region that controls circadian rhythms (suprachiasmatic nuclei, SCN). The human circadian clock runs with a genetically determined period typically slightly longer than the 24h length of the solar cycle, according to which social, conventional time is set. In healthy individuals, exposure to daylight during the day is sufficient to synchronize this internal time through phase shifts with the solar cycle, thus adjusting the length of its endogenous period. Disrupted synchronization between the master pacemaker in the SCN and peripheral clocks in other brain structures and peripheral organs, or inefficient light synchronization, tend to be concomitant phenomena of many psychiatric and other illnesses, and may be the primary cause of some disorders. In these cases, phototherapy acts as an amplified synchronization signal that modifies the phase, period and amplitude of circadian rhythms and, as a result, ameliorates symptoms and accelerates the treatment of psychiatric disorders in particular. For people with disrupted circadian systems, the phase-response curve is a useful tool to help find the optimal time for phototherapy to achieve the maximum expected effect. Since morning light is essential for humans to adjust the internal long period, morning hours also tend to be the first choice for the application of light intervention.
In addition to the synchronizing effect of phototherapy on the circadian system, light also has a direct effect on brain structures that regulate cognitive function and mood. In particular, ipRGCs affect the perihabellar nucleus, connected to the lateral habenula, whose neuronal activity is significantly altered in patients with depression and other forms of psychiatric illness. Light also directly influences serotonergic, dopaminergic, and noradrenergic neurotransmission via ipRGCs and their indirect impact on relevant neurons in the brainstem (via the lateral habenula and the dorsomedial nucleus of the hypothalamus).

Application of chronobiological phototherapy

Bright light has been used in the treatment of the mentally ill since ancient times. Phototherapy (chronobiological phototherapy, bright light therapy, BLT) as a scientific and therapeutic method began to be used in the 1980s in the scientific and therapeutic treatment of seasonal affective disorder (SAD). Today, it is considered the go-to treatment for this diagnosis, and meta-analyses confirm its effectiveness even in non-seasonal depression, with an effectiveness comparable to that of antidepressants. Light therapy can also be combined with other psychological or pharmacological treatments. Unlike conventionally used pharmacotherapy, phototherapy produces significant improvement within days and can therefore help to bridge the long latency of onset of action of antidepressants. In addition to the treatment of depression, phototherapy is also considered an effective method of treating circadian rhythm sleep disorders and has been successfully applied to a number of other diagnoses – bipolar depression, premenstrual dysphoric disorder, perinatal depression, borderline personality disorder with comorbid depression, Parkinson’s disease, Alzheimer’s disease, etc.

The development and application methodology of this device were created in cooperation with the CTU UCEEB and the NIMH in the project No. FW02020025 funded with state support from the Technology Agency of the Czech Republic and the Ministry of Industry and Trade of the Czech Republic within the TREND Programme.