Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle

Curr Biol · 2013 · Mixed · Low evidence

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MixedLow evidencePaper 6812

Study snapshot

PublicationCurr Biol · 2013

Study typeother

Reported directionMixed

Evidence ratingLow

Abstract

The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior however, evolved in the natural light-dark cycle[1] and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14h40min:9h20min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.

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AI evidence extraction

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Study typeother

Effect directionmixed

Populationhumans

Sample sizeNot supplied

Exposuresummer natural 14h40min:9h20min light-dark cycle camping; exposure to only natural light

Evidence strengthLowConfidence: 84% · Peer reviewed: yes

Main findings

Electrical lighting and the constructed environment were associated with reduced daytime sunlight exposure, increased light exposure after sunset, and delayed circadian timing compared with a summer camping condition under a natural light-dark cycle. After exposure to only natural light, the internal circadian clock synchronized to solar time, with biological night beginning at sunset and ending before wake time just after sunrise; later chronotypes showed larger circadian advances.

Outcomes measured

circadian clock timing
light exposure during day and after sunset
timing of internal biological night
chronotype-related circadian phase shifts
sleep schedules

View raw extracted JSON

{
  "study_type": "other",
  "exposure": {
    "band": null,
    "source": "other",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": "summer natural 14h40min:9h20min light-dark cycle camping; exposure to only natural light"
  },
  "population": "humans",
  "sample_size": null,
  "outcomes": [
    "circadian clock timing",
    "light exposure during day and after sunset",
    "timing of internal biological night",
    "chronotype-related circadian phase shifts",
    "sleep schedules"
  ],
  "main_findings": "Electrical lighting and the constructed environment were associated with reduced daytime sunlight exposure, increased light exposure after sunset, and delayed circadian timing compared with a summer camping condition under a natural light-dark cycle. After exposure to only natural light, the internal circadian clock synchronized to solar time, with biological night beginning at sunset and ending before wake time just after sunrise; later chronotypes showed larger circadian advances.",
  "effect_direction": "mixed",
  "limitations": [],
  "evidence_strength": "low",
  "confidence": 0.84,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "circadian clock",
    "light-dark cycle",
    "electrical lighting",
    "natural light",
    "camping",
    "chronotype",
    "sleep schedule",
    "solar time",
    "biological night"
  ],
  "suggested_hubs": []
}

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AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.

Identifiers and source

DOI: 10.1016/j.cub.2013.06.039
PubMed: 23910656

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