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Light therapy and biological rhythms

Circadian oscillations command the organism of living beings. In humans, these oscillations are controlled by the suprachiasmatic nucleus (SCN) located in the hypothalamus. Thus, the SCN regulates the expression of genes in all peripheral clocks, directing the control of all body functions, such as the production of immune cells, hormone secretion and regulation of the sleep-wake cycle. For more information, read https://www.condorinst.com.br/o-que-ritmo-circadiano/.

The sleep-wake cycle is regulated by several synchronizers such as physical exercise, social work and study routines, food and light. Melanopsins - photoreceptor cells located in the retina - capture light and transmit this information to the SCN through the retinohypothalamic tract. Thus, light is the most powerful information for the regulation of circadian rhythms

Human beings have adapted to light and the organism understands that during the day it is the moment of wakefulness and, during the night, when there is no light, it is a moment of rest. This regulation is complex and depends on the activation of several genes and hormones. Thus, when the sleep-wake cycle is misaligned with the light-dark cycle, mood disorders, cardiometabolic diseases such as diabetes and hypertension, obesity, cardiovascular diseases, immunological diseases and reduced sleep quality occur - which is why achieving synchrony is fundamentalFISHBEIN; KNUTSON; ZEE, 2021However, some individuals suffer from rhythm disorders, which significantly interfere with their personal, social and occupational lives. These disorders can be treated through behavioral interventions (exposure to light, regulation of exercise and eating schedules) and pharmacological interventions (melatonin and melatonin agonists such as ramelteon and tasimelteon). In this article, we will focus on behavioral treatment and specifically light therapy.

Interestingly, the light stimulus presents different responses in the circadian timing system depending on the time of its applicationGOOLEY, 2008). For example, when applied in the morning, it causes an advance in the phase of endogenous rhythms, which makes sleep occur earlier. On the other hand, when applied during the beginning of the biological night, it causes a phase delay, making sleep onset later.

Delayed sleep phase syndrome

In delayed sleep phase syndrome, patients have delayed sleep onset times, at times that are incompatible with what is required by society (initiating sleep between 2:00 am and 6:00 am). Thus, these patients have difficulty getting up and performing activities at conventional times. In addition, sleep restriction usually occurs and, consequently, daytime sleepiness that impairs day-to-day performance. The treatment consists of applying light early in the morning, right after waking up. This will advance the sleep phase. Likewise, it is important to avoid light stimuli during the beginning of the biological nightROSENTHAL; JOSEPH-VANDERPOOL; LEVENDOSKY; JOHNSTON et al., 1990)

Advanced sleep phase syndrome

This disorder is characterized by the significant advancement of sleep periods. Thus, these patients sleep between 6:00 pm and 8:00 pm, and wake up between 1:00 am and 3:00 am. For this reason, in the late afternoon they are drowsy. The treatment consists of the application of light at the beginning of the biological night, in addition to avoiding exposure to light stimuli at the beginning of the night. Not only that, the patient should avoid exposure to light during dawn/morning to avoid undesirable phase advances.

Free course

It occurs when the individual is desynchronized with the light-dark cycle, and the perceived effect is the delay of the sleep phase each day (generally 5 to 15 minutes per day). This disorder is present almost exclusively in visually impaired individuals with optic nerve damageSACK; LEWY; BLOOD; KEITH et al., 1992pathway of light synchronization due to tumors in the central nervous system, reduced sensitivity of the circadian system to light, or inefficient light exposure. In free-course studies in patients with a normal visual system, light exposure was performed right after waking up in order to synchronize the sleep-wake cycle, and the objective was successfully achievedHOBAN; SACK; LEWY; MILLER et al., 1989).

Irregularity of the sleep-wake cycle

In patients with this disorder, the sleep-wake cycle occurs in a disorganized 24-hour cycle, with frequent napping, insomnia, and excessive daytime sleepiness. Thus, the sleep-wake cycle cannot be completely defined in these patientsGOOLEY, 2008). This disorder usually results from neurological problems (dementia, brain injury, or individuals with intellectual disabilities) or inadequate exposure to circadian synchronizers (light, exercise, food, social engagements). In addition, it can also occur in the elderly or patients with dementia who live in institutions (e.g., nursing homes) due to constant light conditions and unregulated social schedules.

The main goal of the treatment is to consolidate sleep during the night and wakefulness during the day. Thus, exposure to light during the day should be increased, as well as avoiding light at night. In addition, physical exercise and socialization should be encouraged during the light cycle of the dayZEE; VITIELLO, 2009).

Jet lag

This transient disorder occurs after transmeridional voyages that cross more than 3 time zones. The symptoms stem from the desynchronization between the internal timing system and the time at the destination location. It usually takes 1 day for each time zone crossed to adapt to the new destination. Symptoms include gastrointestinal disturbances, excessive daytime sleepiness, insomnia, difficulty waking up in the morning, reduced cognitive performance, and mood swings.SAMUELS, 2012).

Light can be used to accelerate adaptation and reduce symptoms of jet lag. For this, one must consider the direction of travel and the desired adaptation (phase delay or advance). In general, on trips to the East, what is desired is a phase advance, requiring the application of light stimulation as soon as you wake up. On the other hand, on trips to the west, what is desired is the phase delay, so the application of the light stimulus is recommended at nightSACK; AUCKLEY; AUGER; CARSKADON et al., 2007).

How to assess rhythm disorders

For the evaluation and analysis of rhythms, evaluation through actigraphy - which is the gold standard technique for evaluating biological rhythms - can be performed. The actigraph is a piece of equipment similar to a wristwatch and contains activity, light, and temperature sensors. CONDOR's software has automatic calculations to analyze specific variables of the circadian rhythm, such as non-parametric analysis (L5, M10, IV and IS), spectrogram, periodogram and cosinor. It is also possible to analyze sleep-related variables, such as total sleep time, sleep onset latency, sleep efficiency and awakenings after sleep onset. With this technique, it is possible to analyze the patient for long periods, being ideal to verify if the intervention adopted for the treatment of the observed disorder is generating the expected results.

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