"Did you pull an all-nighter again, Toma?"
Kana said with an exasperated look.
Toma rubbed his eyes. "The experiment was interesting, I got carried away."
"That's bad for you."
Rei spoke up. "It disrupts your biological clock. Circadian rhythm is a gene network operating on a 24-hour cycle."
"Genes are a clock?"
"Yes. Transcription factors called CLOCK and BMAL1 activate at night."
Milia added. "These promote expression of Per and Cry genes."
"Then what happens?" Toma showed interest.
"When Per and Cry proteins accumulate, they return to the nucleus and suppress CLOCK-BMAL1 activity."
Kana drew in her notebook. "A negative feedback loop?"
"Correct. Self-inhibition. This loop takes about 24 hours."
"Why 24 hours?"
Rei explained. "Per protein degradation rate, phosphorylation timing, nuclear translocation speed. These are adjusted to make about 24 hours."
Milia continued. "An enzyme called casein kinase phosphorylates Per. This becomes a signal for degradation."
"What if this enzyme works faster?"
"The cycle shortens. Actually, mutations can change the period."
Toma became serious. "Is my biological clock broken?"
"Not broken. But it may be less easily reset."
"Reset?"
Rei answered. "Light is the reset signal. Morning light resets clock genes."
"How?"
"Special retinal cells detect blue light. That signal reaches a brain region called the suprachiasmatic nucleus."
Milia added. "The suprachiasmatic nucleus functions as the master clock, synchronizing biological clocks throughout the body."
Kana understood. "Bathing in morning sunlight resets the biological clock. That's why regular lifestyle is important."
"Exactly," Rei nodded.
Toma asked. "What if exposed to bright light at night?"
"The clock delays. The brain mistakes it for daytime."
"Like smartphone screens?"
"They have an effect. Blue light is especially powerful."
Milia's face became serious. "Disruption of the biological clock also affects metabolism. Risk of obesity, diabetes, and cancer increases."
"That serious?" Kana was surprised.
"Cell division, DNA repair, metabolic enzyme activity. All depend on time of day."
Rei gave an example. "Anticancer drug efficacy also varies with administration time. There's a field called chronopharmacology."
"Effectiveness differs by time?"
"Yes. Because the biological clock controls enzyme amounts."
Toma sighed. "Maybe I should stop pulling all-nighters."
"Sleep is also controlled by clock genes," Milia said. "Mice lacking Bmal1 have disrupted sleep rhythms."
"Sleep is determined at the gene level."
Kana summarized. "The biological clock is a gene feedback loop. Reset by light, it controls whole-body functions on a 24-hour cycle."
"Perfect," Rei acknowledged.
Toma said, "Cells don't sleep, but they have rhythm. Ignoring that rhythm creates a night when cells can't sleep."
"Poetic, but also scientifically accurate."
Milia smiled. "Let's sleep early tonight and bathe in tomorrow's morning sun."
"Agreed," Kana said.
The four left the lab, feeling the wonder of biological clocks.