Circadian rhythms are near-24-hour biological cycles that regulate sleep-wake timing, hormone secretion, body temperature, metabolism, and many other physiological processes. These rhythms are generated by an internal biological clock in the suprachiasmatic nucleus of the hypothalamus.

Bright bedroom interior with sunlight streaming through window — Circadian rhythms are 24-hour internal clocks that regulate sleep-wake cycles, hormone release, body temperature, and metabolism. Source: Unsplash.

The Molecular Clock

At the molecular level, circadian rhythms are generated by transcription-translation feedback loops. The core loop involves CLOCK and BMAL1 proteins that bind to E-box elements in the promoter regions of Period (Per1, Per2, Per3) and Cryptochrome (Cry1, Cry2) genes. PER and CRY proteins accumulate in the cytoplasm, form heterodimers, translocate to the nucleus, and inhibit CLOCK-BMAL1 activity, repressing their own transcription. As PER and CRY proteins degrade, CLOCK-BMAL1 activity resumes, and the cycle repeats approximately every 24 hours. This molecular clock exists in virtually every cell in the body, with the SCN acting as the master pacemaker that synchronizes peripheral clocks.

Light Entrainment

The primary environmental cue (zeitgeber) for the circadian system is light. The SCN receives light input from intrinsically photosensitive retinal ganglion cells containing melanopsin, which is maximally sensitive to blue light (460-480 nm). This pathway bypasses the visual system — light entrains the clock even in blind individuals with intact ipRGCs. Morning light exposure advances the circadian clock (shifts it earlier), helping you wake up earlier and fall asleep earlier. Evening light exposure delays the clock (shifts it later), pushing sleep and wake times later. The phase response curve (PRC) describes the magnitude and direction of phase shifts depending on the timing of light exposure. Maximal phase advance occurs with light exposure in the early morning (approximately 2-3 hours before habitual wake time). Maximal phase delay occurs with light exposure in the evening (approximately 2-3 hours before habitual bedtime).

Melatonin

Melatonin is the hormone of darkness. The pineal gland secretes melatonin in response to darkness, with onset approximately 2-3 hours before habitual bedtime (dim light melatonin onset, DLMO). Melatonin binds to MT1 and MT2 receptors in the SCN, promoting sleep onset and phase-shifting the circadian clock. Light suppresses melatonin production — even brief exposure to bright light (over 200 lux) can suppress melatonin by 50% or more. Evening melatonin administration (0.5-5 mg, 1-3 hours before bedtime) advances the circadian clock (especially for phase delay disorders). Morning melatonin administration delays the clock. Low-dose melatonin (0.5 mg) may be more effective for phase shifting than high doses, and timing is more critical than dose.

Circadian Rhythm Sleep-Wake Disorders

Circadian rhythm disorders disrupt the alignment between the internal body clock and the external environment, affecting sleep timing and quality. Source: Unsplash.

Delayed Sleep-Wake Phase Disorder

DSWPD is characterized by a persistent delay in the circadian clock, resulting in sleep onset and wake times that are 2-6 hours later than desired. Patients cannot fall asleep until 2-6 AM and have extreme difficulty waking in the morning for school or work. On free days, they sleep well with normal quality when allowed to follow their delayed schedule. Prevalence is 7-16% in adolescents and young adults (common during adolescence due to natural phase delay). Causes include genetic predisposition (PER3 gene variants), behavioral factors (evening light exposure, irregular schedules), and loss of morning light entrainment. Treatment includes morning bright light therapy (30-60 minutes of 2,500-10,000 lux within 30 minutes of waking), timed low-dose melatonin (0.5-1 mg, 5-7 hours before desired bedtime), and chronotherapy (gradually shifting bedtime earlier by 15-30 minutes per day or by advancing 3 hours per day around the clock — the “darkness therapy” approach).

Advanced Sleep-Wake Phase Disorder

ASWPD is characterized by a persistent advance of the circadian clock — patients fall asleep at 6-9 PM and wake at 2-5 AM. It is more common in older adults (prevalence approximately 1% in middle-aged adults). Treatment includes evening bright light therapy (for 30-60 minutes in the early evening to delay the clock) and timed melatonin (low dose in the morning, though careful timing is required to avoid excessive phase delay).

Shift Work Disorder

Shift work disorder affects individuals who work during the normal sleep period (night shifts, rotating shifts, early morning shifts). It affects 10-40% of shift workers. Symptoms include excessive sleepiness during work hours, insomnia during attempted daytime sleep, and impaired performance and increased accident risk. The misalignment between the circadian clock (which is trying to maintain a daytime orientation) and the imposed sleep-wake schedule causes these symptoms. Management strategies include strategic light exposure (bright light during the first part of the shift, avoid light exposure during the commute home — wear blue-blocking glasses), scheduled napping (30-60 minutes before the shift), controlled caffeine use (early in the shift only), sleep environment (dark, cool, quiet bedroom, blackout curtains, white noise, consistent sleep schedule even on off days), and melatonin (3-5 mg before daytime sleep).

⚠ Clinical Correlation

Jet lag results from rapid travel across time zones when the internal clock cannot immediately synchronize with the new environment. The general rule: it takes approximately one day per time zone crossed to fully adjust. Westward travel is easier (phase delay — the clock lengthens, requiring evening light and later bedtimes). Eastward travel is harder (phase advance — the clock shortens, requiring morning light and earlier bedtimes). Management: upon arrival at destination, adopt the local schedule immediately. Seek morning light after eastward travel (facilitates phase advance). Seek afternoon/evening light after westward travel (facilitates phase delay). Take short-acting hypnotics (zolpidem) for the first 2-3 nights if needed. Use caffeine strategically in the morning but avoid after 2 PM. For short trips (1-3 days), maintain home time zone schedule when possible. Melatonin (0.5-5 mg at destination bedtime) may help. The American Academy of Sleep Medicine recommends timed light exposure as the jet lag treatment of choice.

Non-24-Hour Sleep-Wake Rhythm Disorder

Non-24 is characterized by a circadian period that is longer than 24 hours (typically 24.5-25 hours), causing the sleep-wake cycle to drift progressively later each day. It is highly prevalent in blind individuals (50-80% of totally blind people) and rare in sighted individuals. Treatment for blind individuals includes tasimelteon (a melatonin receptor agonist approved for Non-24) and timed melatonin. For sighted individuals, treatment includes bright light therapy and behavioral interventions.

Chronotypes

Individual differences in circadian timing determine chronotype (morningness-eveningness). Morning types (larks) prefer early bedtimes and early wake times, peak performance in the morning, and have an earlier DLMO and core temperature nadir. Evening types (owls) prefer late bedtimes and late wake times, peak performance in the evening, and have a later DLMO. Chronotype is 30-50% heritable and changes with age (children are morning types, adolescents shift late, circadian phase gradually advances with aging). Understanding chronotype is important for scheduling school and work, timing medical treatments (chronotherapy), and optimizing athletic and cognitive performance.

Summary

Circadian rhythms are generated by molecular transcription-translation feedback loops in every cell, orchestrated by the SCN master clock. Light is the primary zeitgeber, with phase advances from morning light and phase delays from evening light. Circadian rhythm disorders include DSWPD (delayed clock, treated with morning light and evening melatonin), ASWPD (advanced clock, treated with evening light and morning melatonin), shift work disorder (managed with strategic light exposure, napping, and environmental optimization), jet lag (managed with immediate adoption of local schedule and timed light exposure), and Non-24 (common in blind individuals, treated with tasimelteon). Individual chronotypes range from morning larks to evening owls.