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https://ntp.niehs.nih.gov/go/NSW_LAN

NTP Review of Shift Work at Night, Light at Night, and Circadian Disruption

Black and white drawing of person at laptop working late into the night

Topic Overview

Status: NTP cancer hazard assessments
Conclusion(s): NTP concluded that (1) persistent night shift work can cause breast cancer in women and (2) certain lighting conditions may be linked to cancer.

Background Information

Cell phone illumination in the dark of night

Modern electric lighting enables people to work, sleep, eat, and receive services throughout the 24-hour day. As a result, many individuals encounter new patterns and types of electrical light, change the timing of their meals, and experience sleep disturbances and other stresses. Those factors can disrupt circadian rhythms and possibly lead to adverse health effects such as cancer.

NTP studied two exposure scenarios related to modern lighting practices.

1. Night shift work

Night shift work is defined as working three hours between 12 and 5am. It is a complex exposure scenario and includes the following.

  • Black and white drawing of person at laptop working late into the night

    Extreme light at night (LAN)

  • Cartoon drawing of a distressed man trying and failing to sleep

    Sleep disruption

  • A plate of fruit and nuts arranged to mimic an analog clock

    Altered meal timing

  • Clipart collection showing a frazzled woman

    Stress and behaviors


2. Sources of electrical light at night

  • Map of the United States showing outdoor light at night

    Outdoor light

  • Man using his phone in bed with his face illuminated by the screen

    Self-luminous electronics

  • A young child reading by lamp light before bed

    Light before bedtime or sleeping

Why is it important to study night shift work and light at night?

According to the Centers for Disease Control and Prevention, more than 10 million adults in the U.S. frequently work night shifts. Frequent night shift work is more common among men, blacks, and non-Hispanics.

Night shift work may contribute to almost 12,000 new breast cancer cases per year in the U.S. The link between night shift work and breast cancer among black women is a research gap.

Exposure to indoor electric lighting is nearly ubiquitous in our society. Findings from the National Sleep Foundation show that 90% of Americans use some type of electronic device a few nights per week within one hour of bedtime. In 2016, satellite imaging data of the Earth at night (see Figure 5) indicated that more than 99% of the U.S. population lived under light-polluted skies at night.

NTP Evaluation

NTP used systematic review methods and established criteria to reach its cancer hazard conclusions. These conclusions are based on scientific judgment with consideration given to all relevant information.

What did the evaluation find?

High confidence that persistent night shift work that disrupts circadian rhythms can cause breast cancer in women and may cause prostate cancer in men. Persistent night shift work is defined as frequent and long-term night shift work, especially beginning in early adulthood.

  • The exact conditions of persistent night shift work are hard to define as duration and frequency may depend on their specific combination.

Moderate confidence for a causal relationship between human cancer and certain lighting conditions that cause circadian disruption. Certain lighting conditions are defined as excessive LAN exposure combined with insufficient daylight exposure.

  • The lighting conditions at night that lead to circadian disruption depend on a combination of light characteristics such as wavelength, exposure duration, light intensity or levels, and timing.
FAQs
circadian disruption clock

The circadian clock
Figure adapted from Nobel Prize 2017,
with permission.

Q: How does circadian disruption work?

Sunlight is the major external signal that synchronizes the master circadian clock to natural light-dark cycles we experience daily. To keep biological rhythms coordinated (see figure for example of daily rhythms), the master clock, which is located in the brain, sends signals to a large network of peripheral clocks that are in almost every cell of the body. Exposure to too much light at night, especially during times when people are typically asleep, is thought to contribute to circadian disruption and increased risk of cancer.

Q: How were the conclusions on persistent night shift work and certain lighting conditions reached?

NTP used systematic review methods to identify studies, evaluate study quality, and integrate evidence across studies.

For each exposure scenario (night shift work or LAN), NTP integrated the evidence from human and non-human studies of exposure and cancer, studies of circadian disruption, and studies of mechanisms (e.g., biological effects) to reach our conclusions.

exposure scenarios of night shift work

Using established criteria, we reached cancer hazard conclusions for each exposure scenario regarding the strength of the evidence from cancer epidemiology studies and for the overall conclusions. Conclusions were based on scientific judgment with consideration given to all relevant information. Subject-specific technical advisors served on a workshop panel to provide input on methods and scientific issues, and reviewed sections of the draft monograph.

An eight-member peer-review panel with discipline and subject-specific expertise concurred with NTP’s hazard conclusions for the two exposure scenarios that cause circadian disruption: (1) persistent night shift work and (2) certain light conditions.

Q: What evidence supports NTP hazard conclusions for persistent night shift work?

This conclusion is based on sufficient evidence of carcinogenicity from the collective body of evidence of cancer epidemiological studies in human populations and mechanistic studies in humans and in experimental animals.

Numerous epidemiology studies conducted in different geographical locations and among workers with different occupations provide evidence of increased breast cancer risk among night shift workers. The strongest evidence is for workers with “persistent” exposure, i.e., longer duration, higher frequency, and beginning night shift work at a younger age, when breast tissue may be more susceptible to environmental exposures.

The exact conditions of persistent night shift work are hard to define, as duration and frequency may depend on their specific combination. For example, an analysis of pooled data from five epidemiology studies found the highest risk of breast cancer among premenopausal women who worked at least 3 nights per week for at least 10 years. Another study found that younger women who worked night shifts at least three times a month for at least 20 years had more than a two-fold increased risk of developing breast cancer. Cancer risk may also depend on other factors such as the ability to adapt to shift work or traits that increase susceptibility to cancer.

Mechanistic studies demonstrate that circadian disruption plays a role in cancer development (e.g., human studies showing night shift workers suffer from circadian disruption, and animal and other studies showing that circadian disruption is linked to cancer). Animal studies also show that modeling shift work or jet lag by varying light-dark patterns over time promotes mammary gland and other types of cancer. Other studies show that night shift work can lead to the same type of biological effects in humans and animals that are typically induced by known carcinogens.

Q: What evidence supports NTP hazard conclusions for certain lighting conditions?

Studies in experimental animals showed that exposure to light at night can have multiple effects. Such exposure has been shown to cause circadian disruption (as evident by melatonin suppression and alterations in clock genes, for example) and promote proliferation and growth of tumors. Tumor growth is mediated by circadian disruption, among other factors. For example, some studies suggest that tumor promotion is mediated in part by decreased levels of the hormone melatonin; normal melatonin levels are important in preventing tumor growth. Light at night exposure in animal studies is also linked with other biological effects that are associated with cancer and can promote tumor growth.

In humans, exposure to light at night has also been shown to decrease nighttime melatonin levels.

Q: Are there ways to prevent cancer or death associated with these two exposure scenarios?

The National Institute for Occupational Safety and Health (NIOSH) offers several trainings and has prepared a science blog for people who work shift work and long hours, such as nurses or first responders. Additional information is also offered at the Occupational Safety and Health Administration’s website.

A recent international workshop of night shift work experts concluded that night shift work schedules that reduce circadian disruption may reduce breast cancer risk. They recommended that night shift schedules have no more than three consecutive night shifts, shift intervals of at least 11 hours, and shift duration of nine hours or less (Garde et al. 2020).

People using electronics in the evening may want to limit the use of bright screens before bedtime. There are also some apps that can reduce blue light from electronic devices during scheduled hours. Light that includes long wavelengths (red and orange in color) minimizes circadian disruption. Many devices have a night shift mode that reduces blue light emissions.

Documents

Documents for Shift Work at Night, Light at Night, and Circadian Disruption
Date Document
Jan 19, 2012 Federal Register notice requesting public comment on nominated substances (PDF HTML)
Apr 01, 2014 Concept Document
May 07, 2018 RoC Protocol
Aug 24, 2018 Draft RoC Monograph
April 27, 2021 NTP Cancer Hazard Assessment: Night Shift Work and Light at Night (Abstract)

Meetings & Events

Listing of related events
Date Event Event Type Materials
Jun 25, 2013

NTP Board of Scientific Counselors Meeting

Board of Scientific Counselors
  • Agenda
  • Meeting Materials

    NTP Board of Scientific Counselors Meeting Meeting Materials
    June 25, 2013

    Supplemental materials for events, meetings, and workshops prior to 2018 have been archived. These archived materials frequently include presentations, background materials, and public comments. Email us or use our contact form to request a list or copy of archived materials for the following meeting.

     
  • Minutes
Mar 10, 2016

Workshop: Shift Work at Night, Artificial Light at Night, and Circadian Disruption

Webinars & Workshops - RoC
  • Final Documents

    Workshop: Shift Work at Night, Artificial Light at Night, and Circadian Disruption Final Documents
    March 10–11, 2016

    Workshop Report

  • Meeting Materials
  • Videos

    Workshop: Shift Work at Night, Artificial Light at Night, and Circadian Disruption Videos
    March 10–11, 2016

    Some browsers may experience play back issues when accessing these videos. We are working to resolve this issue. Videos may take some minutes to display depending on the size of the video and your Internet connection.

    Videos and Closed Captioning



    Welcome and Introduction - Dr. Kristina Thayer, NIEHS

    Background and Workshop Objectives - Dr. Ruth Lunn

    Overview of circadian disruption, biomarkers, and experimental models - Dr. Fred Turek, Northwestern University


    Friday March 11, 2016

    Presentations & Overview of light at night and its association with circadian disruption - Dr. Mariana Figueiro, Lighting Research Center at Rensselaer Polytechnic Institute

    Overview of studies of health effects and biomarkers of circadian disruption in humans - Dr. Richard Stevens, University of Connecticut

    Overview of studies of health effects and biomarkers of circadian disruption in experimental animals - Dr. Randy Nelson, Ohio State University

    Overview of types and characteristics of shift work and the concept of shift work as a complex exposure scenario - Dr. Roel Vermeulen, Utrecht University, The Netherlands

    Overview of studies of shift workers and health effects and/or biomarkers of circadian disruption in humans - Dr. Johnni Hansen, Danish Cancer Society

    Overview of studies of health effects and biomarkers of circadian disruption in experimental animal model studies of shiftwork or jet lag - Dr. Andrew Coogan, Maynooth University, Ireland


    Discussion

    Circadian Disruption Discussion

    Characteristics of Artificial Light Discussion

    Characteristics of Shift Work Discussion

    Characteristics of Sleep Discussion

    Shift Work at Night, Artificial Light at Night, and Circadian Disruption Discussion

    Artificial Light at Night Closing Discussion

     
Oct 05, 2018

Peer Review of the Draft Report on Carcinogens Monograph on Night Shift Work and Light at Night

Expert Panels - RoC Monographs

Supplemental materials for events, meetings, and workshops prior to 2018 have been archived. These archived materials frequently include presentations, background materials, and public comments. Email us or use our contact form to request a list or copy of archived materials for the following meeting.

Public Comments

List of Public Comments for the Report on Carcinogens
Date Received Commenter(s) Affiliation In Response To
Feb 08, 2012 Robert Wagner No Affiliation Listed Jan 19, 2012 Federal Register notice
Feb 20, 2012 Stephen Pauley No Affiliation Listed Jan 19, 2012 Federal Register notice
Mar 01, 2012 Julie Schaar No Affiliation Listed Jan 19, 2012 Federal Register notice

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