Circadian Rhythm
Your internal 24-hour clock. Nearly every cell in your body runs on a circadian rhythm, coordinating when to be active, when to rest, when to repair. Light is the master signal that keeps this orchestra in sync—and modern life constantly disrupts it.
What Follows Circadian Rhythm
Sleep-Wake Cycle
Melatonin rises at night, cortisol rises in morning. Body prepares for activity or rest.
Body Temperature
Drops at night for sleep onset. Rises before waking. Lowest around 4 AM.
Hormone Release
Growth hormone peaks during deep sleep. Testosterone highest in morning. Insulin sensitivity varies.
Metabolism
Blood sugar handling best in morning. Late eating = worse glucose response. Same food, different time.
Immune Function
Inflammatory markers peak at night. Immune surveillance varies throughout day.
Gene Expression
Clock genes (BMAL1, CLOCK, PER, CRY) control ~40% of all gene expression patterns.
Light: The Master Signal
Morning Light
- Sunlight through eyes hits specialized ipRGCs
- Signals SCN to stop melatonin production
- Triggers cortisol awakening response
- Sets the "start" of your circadian day
- Earlier light = earlier sleep that night
Evening Light
- Blue light after sunset suppresses melatonin
- Shifts circadian clock later (phase delay)
- Screens, LED lights are especially disruptive
- Dim, warm light in evening is ideal
- Darkness signal triggers sleep preparation
Consequences of Circadian Disruption
Shift work, jet lag, and screen use create chronic circadian misalignment with serious health consequences:
Metabolic Disease
Higher rates of obesity, diabetes, metabolic syndrome in shift workers and night owls.
Cardiovascular Risk
Heart attacks cluster in morning hours. Chronic disruption increases heart disease risk.
Mood & Mental Health
Depression, anxiety, bipolar disorder all linked to circadian disruption.
Cancer Risk
Night shift work classified as "probably carcinogenic" by WHO. Breast cancer risk elevated.
Cognitive Decline
Poor sleep and circadian disruption accelerate cognitive aging and dementia risk.
Immune Dysfunction
Vaccination response worse when sleep-deprived. Infection risk increases.
Optimizing Circadian Health
Morning Sunlight
Get bright light within 1-2 hours of waking. 10-30 min outside. Sets your clock.
Evening Light Control
Dim lights after sunset. Blue-blocking glasses. Night mode on devices. Red/amber lighting.
Consistent Schedule
Same wake time every day—even weekends. Consistency is more important than duration.
Meal Timing
Eat during daylight hours. Avoid eating close to bedtime. Time-restricted eating can help.
Temperature Cues
Cool bedroom for sleep. Morning warmth can help waking. Hot bath 1-2hr before bed.
Exercise Timing
Morning/afternoon exercise reinforces circadian rhythm. Late exercise can delay sleep.