Circadian Rhythm

What it is

The circadian system is a ~24-hour oscillator coordinated by the suprachiasmatic nucleus (SCN) in the hypothalamus, entrained by light and food, and propagated to nearly every cell through tissue-level clocks (Bmal1, Clock, Per, Cry).

Why it matters

• Metabolic rhythms: insulin sensitivity, lipid metabolism, and many enzyme systems are time-of-day dependent.
• Hormones: cortisol peaks in the morning, melatonin at night, growth hormone in early sleep.
• Cell biology: ~10–40% of transcripts in different tissues are circadian-regulated.
• Disease risk: chronic circadian disruption (shift work, social jet lag, late-night light, irregular eating) associates with cardiovascular disease, type-2 diabetes, certain cancers (breast, colorectal), depression, and reduced lifespan in animal models.

Why aging matters here

• Aging itself weakens circadian amplitude — the body’s clock signals get fuzzier.
• This may contribute to sleep fragmentation, metabolic deterioration, and cognitive decline.

What helps

• Morning bright light exposure (anchors the clock).
• Consistent sleep and wake times.
• Eating during daylight hours (early time-restricted eating).
• Limiting late-night light, especially blue spectrum.
• Avoiding food and alcohol close to bedtime.
• Treating sleep disorders (OSA in particular).

Related entries

Sleep optimization, Time-restricted eating, Melatonin.