ABSTRACT

Various physiological systems and behaviors such as the sleep-wake cycle, vigilance, body temperature, and the secretion of certain hormones are governed by a 24-hour cycle called the circadian system. While there are many external stimuli involved the regulation of circadian rhythm, the most powerful environmental stimulus is the daily light-dark cycle. Blind individuals with no light perception develop circadian desynchrony. This leads to non-24-hour sleep-wake rhythm disorder, which is associated with sleep-wake disorders, as well as mood disorders and loss of appetite and gastrointestinal disturbances due to disrupted circadian hormone regulation. As the diagnosis is often delayed because of under-recognition in clinical practice, patients must cope with varying degrees of social and academic dysfunction. Most blind individuals report that non-24-hour sleep-wake rhythm disorder affects them more than blindness. In the treatment of totally blind patients suffering from non-24-hour sleep-wake rhythm disorder, the first-line management is behavioral approaches. Drug therapy includes melatonin and the melatonin agonist tasimelteon. Diagnosing blind individuals’ sleep disorders is also relevant to treatment because they can be improved with the use of melatonin and its analogues or by phototherapy if they have residual vision. Therefore, assessing sleep problems and planning treatment accordingly for individuals presenting with blindness is an important issue for ophthalmologists to keep in mind.

Introduction

All living things have a multitude of biological rhythms occurring at different frequencies and periods, ranging from the cellular level to the physiological and social behavioral levels.¹ In humans, biological rhythm frequencies spanning nearly all segments of time have been described, such as electroencephalogram waves that oscillate by the second, 24-hour sleep-wake rhythms, the weekly pattern of urinary 17-ketosteroid excretion, and other rhythms that occur monthly, annually, or even every 10 years, like the appearance of some sunspots.² Among these biological rhythms, circadian rhythms (referring to a 24-hour period) are perhaps the most well studied. Human physiology and behavior are governed by a 24-hour circadian rhythm. The sleep-wake cycle, attention, behavior patterns, and hormone secretion are just a few examples of biological systems regulated by the circadian cycle. This rhythm is spontaneously adjusted by the suprachiasmatic nucleus in the anterior hypothalamus, which is an internal rhythm regulator.³ In most people, this circadian rhythm is slightly longer than 24 hours and is adjusted daily to the solar rhythm of 24 hours according to environmental cues. The most important environmental cue for synchronization is light. Daily retinal exposure to light is needed to adjust the circadian rhythm to 24 hours.⁴ Except for those with jetlag or shift work, this daily synchronization occurs with no problem in people with good eyesight. However, people with bilateral vision loss or blindness become desynchronized due to the lack of light input to the suprachiasmatic nucleus.⁵

In this review, we aimed to examine the changes in circadian rhythm associated with the lack of light input in blind individuals, as well as the physical and mental consequences of and treatment approaches to these changes, in light of the current literature. To better understand the physiopathology, we first examine the relationship between light, melatonin, and circadian rhythm.

Conclusion

Blind people who cannot receive light input experience symptoms such as insomnia and excessive daytime sleepiness due to disruption of and inability to re-entrain the circadian rhythm. In addition, disruptions in physiological functions and hormone release regulated by the circadian rhythm lead to various adverse consequences in their social, academic, and professional lives. Keeping a sleep diary, obtaining actigraphy measurements, and when necessary, analyzing biochemical parameters are beneficial when diagnosing non-24-hour sleep-wake disorder, which is very rare in the sighted population but common in the blind. Behavioral and pharmacological methods are often effective in the treatment of this disorder. The need to continuously use the drugs that prevent circadian drift is important in terms of considering effectiveness and cost when selecting pharmacological treatment. The diagnosis is often delayed, causing considerable functional losses for blind people, who already face obstacles in many areas of daily life. Diagnosis is also relevant to treatment, as the sleep patterns of blind people can be made more normal through the use of melatonin and its analogues, or phototherapy if they have residual vision. Therefore, assessing sleep problems and planning treatment accordingly for individuals presenting with blindness is an important issue for ophthalmologists to keep in mind.