Lifestyle and Environment · Sleep and Circadian Health
RRM Academy Synopsis
Dim Night Light and Constant Darkness Jeopardize Lab Rodent Health
Dim light at night and constant darkness can disrupt body clocks in laboratory rodents, a 2018 narrative review of animal studies reports. The studies used mice, rats and hamsters. The review links both conditions to depression-like behavior, memory problems and altered metabolism. The author calls for full darkness at night and a regular day-night cycle in animal facilities.
Key Findings
- Dim white light at night of 5 lux, five times brighter than maximal moonlight, was linked in the studies reviewed to reduced sleep efficiency, depression and cognitive impairment in adult rodents.
- Dim light as low as 0.2 lux shifted the daily timing of glucose, lactic acid and corticosterone peaks in rats. Low night light also suppressed melatonin in rats and hamsters.
- A previous study found that 7 days of constant darkness was enough to produce a depressive behavioral phenotype in rats that 1–2 days of a 12:12 light-dark cycle did not reverse.
- Pups raised in constant darkness until weaning showed a depressive-like phenotype in adulthood, even after standard 12:12 light-dark housing.
- Constant dim red light at 0.5–1 lux progressively reduced the amplitude of the sleep-wake rhythm in adult nocturnal rats by decreasing sleep during the rest phase.
Interpretation
The paper is a narrative review by a single author. It summarizes experiments in mice, rats and hamsters, including the author's own. Results varied between studies. Body weight and glucose tolerance under dim night light differed by age at exposure, and the author suggests that differences in methods may explain conflicting results. The experimental findings come from animals. The author recommends 0 lux at night, full reporting of lighting in published methods, and international agreement on light-dark cycles for laboratory animals.
RRM Context
The review states that people who work or travel across shifting lighting cycles can develop irregular rhythms, which increase the risk of chronic health conditions. Preterm births, spontaneous abortion and lower birth weight are among those listed. The review also reports that brief, very dim red light entrained the timing of ovulation in animals kept in constant darkness. Restorative reproductive medicine treats the ovulatory cycle as a sign of whole-body health. A body clock that depends on light is relevant background.
Abstract
The influence of light on mammalian physiology and behavior is due to the entrainment of circadian rhythms complemented with a direct modulation of light that would be unlikely an outcome of circadian system. In mammals, physiological and behavioral circadian rhythms are regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus. This central control allows organisms to predict and anticipate environmental change, as well as to coordinate different rhythmic modalities within an individual. In adult mammals, direct retinal projections to the SCN are responsible for resetting and synchronizing physiological and behavioral rhythms to the light-dark (LD) cycle. Apart from its circadian effects, light also has direct effects on certain biological functions in such a way that the participation of the SCN would not be fundamental for this network. The objective of this review is to increase awareness, within the scientific community and commercial providers, of the fact that laboratory rodents can experience a number of adverse health and welfare outcomes attributed to commonly-used lighting conditions in animal facilities during routine husbandry and scientific procedures, widely considered as "environmentally friendly." There is increasing evidence that exposure to dim light at night, as well as chronic constant darkness, challenges mammalian physiology and behavior resulting in disrupted circadian rhythms, neural death, a depressive-behavioral phenotype, cognitive impairment, and the deregulation of metabolic, physiological, and synaptic plasticity in both the short and long terms. The normal development and good health of laboratory rodents requires cyclical light entrainment, adapted to the solar cycle of day and night, with null light at night and safe illuminating qualities during the day. We therefore recommend increased awareness of the limited information available with regards to lighting conditions, and therefore that lighting protocols must be taken into consideration when designing experiments and duly highlighted in scientific papers. This practice will help to ensure the welfare of laboratory animals and increase the likelihood of producing reliable and reproducible results.
