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Original research
Role of nocturnal light intensity on adaptation to three consecutive night shifts: a counterbalanced crossover study
  1. Erlend Sunde1,
  2. Jelena Mrdalj2,
  3. Torhild Pedersen2,
  4. Eirunn Thun3,
  5. Bjørn Bjorvatn4,5,
  6. Janne Grønli2,
  7. Anette Harris1,
  8. Siri Waage4,5,
  9. Ståle Pallesen1,5
  1. 1 Department of Psychosocial Science, University of Bergen, Bergen, Norway
  2. 2 Department of Biological and Medical Psychology, University of Bergen, Bergen, Norway
  3. 3 Department of Clinical Psychology, University of Bergen, Bergen, Norway
  4. 4 Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway
  5. 5 Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Bergen, Norway
  1. Correspondence to Erlend Sunde, Department of Psychosocial Science, University of Bergen, Christiesgt. 12, 5015 Bergen, Norway; erlend.sunde{at}


Objectives To investigate how a standard ceiling mounted light-emitting diode (LED)-based bright light intervention affected alertness and neurobehavioural performance during three consecutive simulated night shifts, and timing of circadian rhythm after the shifts.

Methods Twenty seven participants (20 females, 21.4±2.1 years; mean±SD) worked three consecutive night shifts (23:00–07:00) under a full-spectrum (4000 K) bright light (900 lx) and a standard light (90 lx) condition in a counterbalanced crossover design (separated by 4 weeks). Subjective alertness (Karolinska Sleepiness Scale) and neurobehavioural performance (Psychomotor Vigilance Task and Digit Symbol Substitution Test) were assessed five times during each shift. Salivary dim-light melatonin onset (DLMO) was assessed before and after the shifts. The simulated night shifts were conducted in a laboratory while the participants slept at home.

Results Subjective alertness and neurobehavioural performance deteriorated during the night shifts in both light conditions. However, bright light significantly reduced alertness and performance decrements as compared with standard light. For a subset of the participants, DLMO was delayed by a mean of 3:17±0:23 (mean±SEM) hours after three night shifts in bright light and by 2:06±0:15 hours in standard light, indicating that bright light causes larger phase delay.

Conclusion Bright light improved performance and alertness during simulated night shifts and improved adaptation to night work. Bright light administered by ceiling mounted LED luminaires has the potential to improve adaptation to night work and reduce the risk of accidents and injuries among night workers.

Trial registration number NCT03203538.

  • shift work
  • neurobehavioural effects
  • circadian rhythms
  • psychology

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  • Contributors The study conception was by SP assisted by ES, BB, JG, AH and SW. All authors were involved in the design of the study. ES, JM, TP and ET collected the data. ES conducted the statistical analysis and drafted the manuscript. All authors have reviewed and approved the final manuscript.

  • Funding The study was supported by a PhD grant for ES and TP, and a Post-Doctoral grant for ET, funded by the Faculty of Psychology, University of Bergen, Norway. The study was carried out in the laboratory of the Faculty of Psychology, University of Bergen, Norway. The set-up of the laboratory was supported by Glamox AS, BU Norge, which provided the LED-luminaires. Glamox AS had no role in the study design, data collection, analysis, decision to publish or preparation of the manuscript. The financial compensation of participants was funded by a surplus from a previous research project conducted by SP.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval The study was conducted according to the Declaration of Helsinki and was approved by the Norwegian Regional Committee for Medical and Health Research Ethics (No. 2016/1903).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available on reasonable request to the corresponding author.