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Morning light does more than brighten a room: it helps reset the body's internal clock, which times alertness, hormones and sleep. The clock runs slightly longer than 24 hours, so without that daily reset it would slowly drift later.

Circadian science / NCBI

Health

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The signal comes from a third kind of light-sensitive cell in the eye, separate from the rods and cones we see with. These cells, identified in 2002 by teams including those of David Berson and Samer Hattar, contain a pigment called melanopsin that responds most to blue light. They report to the suprachiasmatic nucleus, a tiny cluster of nerve cells in the hypothalamus, at the base of the brain, that acts as the body's master clock. A 1999 Harvard study put the human clock's natural cycle at about 24 hours and 11 minutes. Morning light shifts the clock earlier; bright light late in the evening shifts it later and delays melatonin, the hormone that signals night. The clock's molecular machinery earned Jeffrey Hall, Michael Rosbash and Michael Young the 2017 Nobel Prize in Physiology or Medicine.

Türkçe oku
Sabah ışığı yalnızca odayı aydınlatmaz; uyanıklığımızı, hormonlarımızı ve uykumuzu zamanlayan iç saatimizin ayarlanmasına da yardım eder. Bu saatin bir turu 24 saatten biraz uzun sürer; her gün ışıkla ayarlanmasa giderek daha geç saatlere kayardı.

Sinyal, gözdeki üçüncü tür ışığa duyarlı hücrelerden gelir; bunlar görmemizi sağlayan çubuk ve koni hücrelerinden farklıdır. Bu hücreleri 2002'de, David Berson ve Samer Hattar'ın ekiplerinin de aralarında olduğu araştırmacılar tanımladı. Hücrelerde, en çok mavi ışığa tepki veren melanopsin adlı bir pigment bulunur. Sinyallerini, beynin tabanındaki hipotalamusta yer alan suprakiazmatik çekirdeğe iletirler; bu küçük sinir hücresi kümesi bedenin ana saatidir. Harvard'da 1999'da yapılan bir çalışma, insan iç saatinin kendi döngüsünü yaklaşık 24 saat 11 dakika olarak ölçtü. Sabah ışığı iç saati daha erkene çeker; akşam geç saatlerdeki parlak ışık ise onu daha geç saatlere kaydırır ve gecenin geldiğini bildiren melatonin hormonunun salgılanmasını geciktirir. Jeffrey Hall, Michael Rosbash ve Michael Young, iç saatin hücre içindeki işleyişini çözdükleri için 2017 Nobel Fizyoloji veya Tıp Ödülü'nü aldı.

Sources and further reading

Background reading: Circadian rhythm
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