Yanıtını arayan eski bir soru: Niçin uyuruz?

Uyku karmaşık, yüksek derecede organizasyon gösteren, birçok iç ve dış faktörden etkilenen, belli dönemlerinde beyinin uyanıklık kadar aktif olduğu, geri dönüşümlü bir bilinçsizlik ve seçici yanıtsızlık özellikleri gösteren bir süreçtir. Uyku fizyolojisi konusundaki bilgilerimizin hızla artması ve uykuyu oluşturan mekanizmaların çoğunun aydınlatılmasına rağmen uykunun işlevi, diğer deyişle “niçin uyuruz?” sorusunun yanıtı gizemini korumaktadır. Uyku ile ilgili birçok hipotez ileri sürülmesine rağmen bunlardan hiçbirisi uyku araştırmacılarını inandıracak düzeyde deneysel kanıt sağlayamamıştır. Bu hipotezler arasında, enerjinin korunması, beyin detoksifikasyonu, beyin termoregülasyonu, doku yenilenmesi, öğrenme ve bellek oluşumu sayılabilir. REM uykusunun tanımlanmasından sonra bu uyku dönemi ile öğrenme ve bellek pekiştirme arasında bağlantı olduğunu gösteren önemli çalışmalar yapılmıştır. Ancak, uyku olmadan öğrenme mümkün görünmekte, her uyku dönemini mutlaka bellek pekiştirmesinin takip edip etmediği bilinmemekte ve insanlarda bellek pekiştirmesine olan gereksinim uyku oluşması için gerekli ve yeterli bir nedendir denilememektedir. Son dönemde ileri sürülen ilgi çekici hipotezlerden biri de uykunun beyinde sinaptik ağırlığı düzenlediğini savunan sinaptik homeostasis hipotezidir. Bu hipoteze göre uyanıklık süresince beyinde kortikal devrelerde sinaptik potansiasyon olmaktadır; bu potansiasyon yavaş dalga aktivitesi ile ilişkilidir; yavaş dalga aktivitesi sinaptik büyümeyi eski haline getirecek şekilde sinaptik küçülme sağlamaktadır. Elli yıllık araştırmaya rağmen uykunun işlevi ile ilgili söylenebilen ve herkesin hemfikir olduğu tek açıklama şudur: uykululuğu engellemek için uyuruz. Böyle bir bakış açısı, “acıkmayı engellemek için yemek yeriz” ya da “susamamak için su içeriz” açıklamaları kadar umutsuz görünmektedir. Uyku tıbbının kurucularından Allen Rechtschaffen, yukarıdaki açıklama için şöyle söylemektedir: Bu doğru ise uyku, doğanın yaptığı en büyük hatalardan biridir.

An old question seeking for its answer: Why we sleep?

represents a complicated, reversible process affected from many internal and external factors, and characterized by unconsciousness, selective unresponsiveness and high level organization. Brain is as active as wakefulness during certain stages of sleep. Although our knowledge on sleep physiology is rapidly growing and many of the mechanisms generating sleep are elucidated, the function of sleep, in other words, the answer of the question “why we sleep?” remains its mystery. Although many hypotheses oriented towards sleep function rised, none of them has proven experimental evidence sufficient to persuade sleep researchers. These hypotheses include energy conservation, brain detoxification, brain thermoregulation, tissue restoration and learning and memory consolidation. Following the discovery of REM sleep, critical studies demonstrating a relationship between learning, memory consolidation and that sleep stage were performed. However, learning without sleep seems possible; it is not clear that each sleeping period always followed by a memory consolidation process; and it is not easy to say that in humans, need for memory consolidation is necessary and sufficient for sleep generation. One of the recent attractive hypotheses is synaptic homeostasis which suggests that sleep regulates synaptic weight in brain. According to that hypothesis, synaptic potentiation occurs in cortical circuits of brain during wakefulness; this potentiation correlates with slow wave activity; slow wave activity leads to synaptic reduction which recovers synaptic potentiation. Despite research of fifty years, the only explanation everybody agree with is that: we sleep to overcome sleepiness. Such a point of view is as hopeless as explanations like “we eat to overcome hunger” or “we drink to overcome thirst”. Allen Rechtschaffen, one of the founders of sleep medicine, says for the above explanation that: If this is true, sleep is one of the biggest mistakes that nature made.

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