Light is a fundamental aspect of the environment, enabling animals to perceive and respond to visual stimuli as well as regulating circadian rhythms in physiology and behaviour. We have a good understanding of the rod and cone photoreceptors mediating vision, and work over the last 25 years has identified an additional retinal photoreceptor that regulates circadian rhythms and many other non-visual processes. This consists of a subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the blue light sensitive protein melanopsin. To address these differences in visual and circadian responses to light, in humans, standards have been introduced to provide lighting units based upon the different photoreceptors involved. By contrast, lighting in laboratory animal facilities is still based on human vision and not the experience of the animals. Recent guidelines have, for the first time, provided the tools to measure light based upon the differing visual sensitivity of laboratory species such as mice (Mus musculus). Here, we provide an overview of why this is important, for both science and animal welfare, and how researchers and animal care staff can apply these new methods in their work. For the first time, this provides refinement opportunities to optimise lighting based on the animal’s experience of their light environment.

The quality and quantity of nutrition are well-established factors influencing health. However, research on circadian rhythms has revealed that the timing of nutrition is also crucial, as highlighted by the concept of time-restricted feeding (TRF). In laboratory animals, TRF involves limiting daily energy intake to a consistent window of 8–12 h. This approach has been shown to mitigate, and in some cases reverse, the negative metabolic impacts of an obesogenic diet. Specifically, under such diets, TRF has been found to reduce body weight and adiposity; enhance liver, heart and gut health; improve glucose tolerance and blood pressure; decrease the severity of neurodegenerative diseases; and preserve muscle mass and function. When animals are fed a balanced diet, TRF does not lead to significant weight loss but modestly extends lifespan. Moreover, when combined with calorie restriction, TRF can significantly increase median lifespan. Thus, TRF can work synergistically with optimal nutrition quality and quantity to enhance health. The underlying mechanisms of TRF are being uncovered through studies in mice and fruit flies. These studies systematically examine mRNA, proteins and metabolites during the fed and fasted states of TRF, shedding light on the genes and pathways modulated by TRF in major metabolic organs, including the liver, heart, muscle, adipose tissue and gut. The aim of this review is to present the experiments that established TRF as a time-based nutritional intervention for health improvement and to summarise key molecular, physiological and metabolic findings from animal studies.

Host–parasite coevolution resulted in parasites optimizing their life cycle to obtain the most benefit from the host’s behaviour. In passerines, for instance, some parasite groups have found means to match their egg shedding rhythms with the hours of the day when hosts are most active. In other bird groups, such as shorebirds, whose active times are not determined by day–night cycles but by other external factors such as tidal or lunar cycles, it is not yet known whether their endoparasites exhibit any predictable pattern in their egg shedding rhythms. Here we used a simple wild-caught (captive) system, to provide a first approximation of the parasite egg shedding patterns in wintering Rufous-chested dotterels (Zonibyx modestus). We collected faeces every 2 hours over 10 days until completing a 24-hour cycle, which was coupled with continuous video recordings to determine their feeding and drinking habits. Contrary to our expectation, we showed that parasite egg shedding followed a cyclic pattern, characterized by 2 peaks: one at midnight and another in the afternoon. Importantly, this shedding pattern was not related to the birds’ feeding or drinking habits. We discuss possible environmental and physiological cues that parasites might use to trigger egg production, including the potential influence of tidal cycles on our results.

Hickie et al. (2023) pose the question “Are sleep and circadian rhythm disturbances (SCRD) the cause or simply the consequence of depression or other mood disorder sub-types?” and suggest strategies to better understand the role of SCRD in depression. Here, we contribute to the discussion by highlighting state-of-the-art computational omics methods (and the data sets needed to use these methods) which have potential for improving our understanding of the role of circadian biology in mood disorders.

This editorial summarises the clinical relevance of ‘chronopsychiatry’, defined as the interface between circadian science and mental health science. Chronopsychiatry represents a move towards time-variable perspectives on neurobiology and symptoms, with a greater emphasis on chronotherapeutic interventions.

The objective was to examine associations between social jetlag and diet quality among young adults in the US using nationally representative data from the 2017–2018 NHANES survey, and evaluate effect modification by gender and race/ethnicity. Social jetlag was considered ≥2-hour difference in sleep midpoint (median of bedtime and wake time) between weekends and weekdays. Diet quality was assessed with the Healthy Eating Index (HEI)-2015 and its 13 dietary components. Ordinal logistic models were run with diet scores binned into tertiles as the outcome. Models accounted for potential confounders and survey weights. Effect modification by gender and race/ethnicity was examined. The study sample included 1,356 adults aged 20–39 years. 31% of young adults had social jetlag. Overall, there were no associations between social jetlag and diet quality. However, interaction analysis revealed several associations were race-specific (P, interaction<0.05). Among Black adults, social jetlag was associated with lower overall diet quality (OR = 0.4, 95% CI 0.2, 0.8; i.e. less likely to be in higher diet quality tertiles) and more unfavourable scores on Total Vegetables (OR = 0.6, 95% CI 0.3, 1.0) and Added Sugar (i.e. OR = 0.6, 95% CI 0.4, 0.9). For Hispanic adults, social jetlag was associated with worse scores for Sodium (OR = 0.6, 95% CI 0.4, 0.9) However, White adults with social jetlag had better scores of Greens and Beans (OR = 1.9, 95% CI 1.1, 3.2). Within a nationally representative sample of US young adults, social jetlag was related to certain indicators of lower diet quality among Black and Hispanic Americans.

Circadian dysfunction is a core feature of bipolar disorder and may be due, at least in part, to abnormalities of non-visual photoreception. We critically review the evidence for light hypersensitivity in bipolar disorder and discuss how this may shape future research and clinical innovation, with a focus on a possible novel mechanism of action for lithium.

Environmental light-dark cycles play an important role in behavioural and physiological processes. It is essential that laboratory vivaria be designed to properly control the light conditions in which laboratory mice are housed; however, this is not universally the case. Some laboratory vivarium doors are designed with windows, which allow light from the hallways to leak into the housing space during the rodents’ dark phase. Personnel entering and exiting the housing space during the dark phase can also create excessive light leak from brightly illuminated hallways. In this study, we investigated the hypothesis that exposure to dim light at night, as commonly experienced in many laboratory rodent housing spaces, alters mouse (Mus musculus) behaviour. We specifically analysed patterns of locomotor activity, anxiety- and depressive-like responses. Exposure to dim (5 lux) light at night altered home-cage locomotor activity and increased anxiety and some depressive responses among laboratory mice. These results suggest that light conditions can alter mouse behaviour and potentially influence experimental outcomes. Increased care should be taken to properly control light-dark conditions for laboratory animals.

Parasites display a wide range of behaviours that are frequently overlooked in favour of host responses. Understanding these behaviours can improve parasite control through a more precise application or development of new behaviour-based strategies. In aquaculture fish lice are an ongoing problem, infections reduce fishery production and control options are limited. Fish lice are distinct in their ability to survive and swim off hosts, allowing the transmission to multiple fish hosts across their lifespan. Here we assessed the off-host behaviour of Argulus foliaceus (a freshwater fish louse) and observed a diurnal rhythmical pattern in their behaviour. This pattern was lost when lice were exposed to constant darkness, indicating that the behaviour is not endogenously driven. Males were consistently active in light with reduced activity in darkness. In contrast, females were active during light and dark phases with peak activity at the start of dark periods. A. foliaceus was also strongly attracted to a light stimulus, preferring white- and blue-coloured lights over green- or red-coloured lights. Light is a strong driver of fish louse activity and could be used to trap parasites. Aquaculture light regimes could also be altered to reduce parasite attraction and activity.

This study evaluated the potential of circadian measures as early markers of mood disorders subtypes. Patients with bipolar disorders had significantly lower levels and later onset of melatonin secretion than those with unipolar depression. Furthermore, abnormal phase angles between sleep, melatonin and temperature were found in several patients.