Effects of light interventions for adaptation to night work : Simulated night work experiments

Abstract

In modern society, the need for 24-hr operation and services requires some people to work outside normal daytime work hours (i.e. shift work), including the night. For instance, healthcare, police, and transportation, are sectors where night work is common. Exposure to shift work, and particularly night work, can have negative impact on the workers’ health. Especially, sleep is reported to be disturbed among night workers, as they must be awake at times they would normally be sleeping, and sleep at times they would normally be awake. This circadian misalignment of the sleep-wake rhythm may in a long-term perspective lead to ill health and diseases. Also, in a short-term perspective night work may cause adverse effects. Night workers experience increased sleepiness and performance deterioration during night shifts, and especially in the early morning hours, the sleep propensity and performance decrements are high. As such, night work has also been associated with increased risk of accidents and injuries.

Several countermeasures to reduce the adverse impact of night work have been suggested. Common strategies involve scheduled naps and caffein use. However, there is increasing interest in the use of light interventions for eliciting beneficial effects for night workers. Light exposure has the potential to entrain the biological circadian rhythm in humans, and as such can be used to produce circadian adaptation to a night work schedule. In addition, light has acute alerting effects which can reduce alertness deficits and improve performance during the night shift. Such effects rely on several characteristics of the light, such as timing, intensity, and wavelengths (spectral distribution). With the development of light emitting diode (LED) technology, new strategies for illumination of workplaces have emerged.

This thesis is based on three papers using standard ceiling mounted LED-luminaires to administer different light conditions during simulated night shift experiments. The main aim has been to investigate and elucidate how such LED lighting strategies can be used to facilitate adaptation to night work on measures of sleepiness, performance, and circadian rhythm.

In paper 1, the objective was to investigate how a full spectrum (4000 K) bright light (~ 900 lx), compared to a standard light (~ 90 lx), affected alertness and performance during three consecutive simulated night shifts (23:00–07:00 hrs), as well as circadian phase shift after the simulated night shifts. Results indicated that bright light effectively reduces sleepiness, and improves performance during three consecutive night shifts, compared to standard light. Bright light seems to be beneficial in the later parts of the shifts, when sleep propensity is particularly high. For instance, in the later parts of night 2 and 3 it was found that the number of lapses of attention on a vigilance task revealed half as many lapses with bright light, compared to standard light. Furthermore, bright light induced a larger phase delay as compared with standard light, although data were incomplete, hence validation of these findings are needed.

The objective in the second paper was to investigate how short-wavelength monochromatic blue light (λmax = 455 nm), compared to red light (λmax = 625 nm) with similar photon density (~ 2.8 x 1014 photons/cm2/s), affected alertness and task performance during one simulated night shift (23:00–06:45 hrs), as well as circadian phase shift following the night shift. The results in paper 2 suggest that monochromatic blue light reduces sleepiness and improves performance in the later parts of the night shift. Similar to the findings in paper 1, the number of attentional lapses with blue light was half of that seen with red light. Blue light also led to a larger phase delay of the circadian rhythm. There were indications of improved visual comfort with blue light, although both light conditions overall produced visual discomfort.

In the third paper the main aims were to investigate how polychromatic blue-enriched white light (7000 K; ~ 200 lx), compared to warm white light (2500 K) of similar photon density (~ 1.6 x 1014 photons/cm2/s), affected alertness and performance during three consecutive simulated night shifts (23:00–06:45 hrs), as well as circadian adaptation to the night work schedule. The results indicated minor, yet beneficial effects of 7000 K light compared to 2500 K light, mainly in terms of fewer performance errors on a vigilance task in the end of night 1 and 2. No significant difference in terms of circadian phase shifts were found between these two light conditions.

In conclusion, the papers suggest that standard ceiling mounted LED-luminaires have the potential to produce light conditions that may facilitate adaptation to night work. Paper 1 suggests that bright light improves performance and reduces sleepiness during three consecutive simulated night shifts. Results from paper 2 indicate that short-wavelength blue light improves performance, reduces sleepiness, and causes a larger phase delay than long-wavelength red light during one simulated night shift. Paper 3 indicates that using polychromatic blue-enriched white light has minor, yet beneficial effects on performance measures, compared to warm white light during three consecutive simulated night shifts. Further research is needed to validate and support the findings and investigate the impact and feasibility of similar light conditions in real-life workplaces. Future research should also explore more light conditions that can be favourable for night workers, in order to develop recommendations for illumination of night workers workplaces. Moreover, there is a need to elucidate potential long-term adverse health impacts of exposure to LED lighting.