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Table 2 Overview of identified models associating health with time-of-day (such as in shift work), working time duration (such as in extended working hours), and nonstandard working schedules (where time-of-day and working time duration are combined)

From: Nonstandard working schedules and health: the systematic search for a comprehensive model

Study Schedule Model summary Health outcome
1. Rutenfranz [52] Shift work The association between objective stress (phase shifting of work and sleep) and strain (lowered well-being and disease) is mediated by intervening variables(e.g. personality, physiological adaptability). The intervening variables determine whether a person can cope with shift work or whether lowered well-being may develop into disease. Complaints and disease
2. Haider [31] Shift work 1. Model by Åkerstedt et al. (1977). Adjustment of the circadian rhythm to working hour requirements may lead to complaints and social role conflicts. Depending on exposure, health state, and personality, this may in turn lead to poor attitudes, absenteeism, and illness. 1. Digestive problems, illness, absenteeism
2. Destabilisation hypothesis from Kundi et al. (1979). Adaptation to shift work is central in the maintenance of shift worker health, accomplished by a stable interaction between family life, sleep behaviour, and attitudes towards shift work. Personal factors, work situation, and social environment modify the interaction. An unstable interaction may, over time via a sensitisation phase, lead to health complaints (accumulation phase). 2. General health problems
3. Folkard [20] Shift work Shift work may affect three interrelated life domains: the biological domain (circadian disruption), medical domain (short-term health consequences), and social domain (social and family activities). Symptoms in these domains may lead to general feelings of malaise in susceptible individuals. General feelings of malaise
4. Monk [86] Shift work Shift work is seen as a source of stress, due to a reversed sleep/wake cycle, that must be coped with. A triad of coping factors—biological clock, social/domestic, and sleep— are described that influence tolerance to shift work. Sleep and stomach complaints, malaise
5. Kundi [37] Shift work Destabilisation theory of shift work, adapted from Kundi et al.(1979) and Haider et al. (1981). The model describes a complex dynamic interaction between three activity spheres (work, family, and recreation) that need to stay in equilibrium to preserve health (adaptation phase). Personality traits, attributions of the social environment, and work situation play a mediating role between the interacting spheres. An unstable interaction between the spheres may, over time, lead to health complaints (accumulation phase) via sensitisation phase. Health state/complaints
6. Knutsson [43] Shift work Three inter-related pathways leading from shift work to coronary heart disease are described: Coronary heart disease
 1. Mismatch of circadian rhythms.
 2. Psychosocial factors lead to stress-induced changes, e.g.lipoprotein disturbances.
 3. Behavioural and life-style changes (coping) (e.g. diet, smoking).
7. Knutsson & Bøggild [38] Shift work Adjusted slightly from Knutsson [43]; it adds three ways in which circadian rhythms can be mismatched:  
 1. A phase shift of circadian rhythm relative to the day-night cycle.
 2. Desynchronisation of different internal body rhythms.
 3. Reduced rhythm amplitude.
8. Olsson [53] Shift work Model based on the transactional psychological theory of stress and coping. Stress occurs when there is an imbalance between the person’s resources and the appraised demands from the environment (occupational shift work stressors, non-occupational stressors). Passive coping styles may lead to poor health when stressors exceed resources. Active coping styles maintain health. Personal factors influence the balance between appraisal, stress, and coping. Poor health: mental load, symptoms, absence, well-being
9. Tepas & Mahan [39] Night work Night work induces acute sleep loss that accumulateds to ‘total acute sleep loss’ with consecutive shifts. Continued night work over the years may lead to chronic sleep deprivation, which in turn can lead to biological deficiency and medical disorders. Shift work tolerant workers are those able to make suitable changes in lifestyle and values. Medical disorders, sickness and death
10. Folkard [51] Shift work Shift system features, influenced by individual and situational differences, may disturb biological rhythms, sleep, and family/social life. These disturbances may affect mood. Success of coping strategies determine whether acute effects develop into chronic mental health problems, which in turn may develop into negative physical health. Chronic mental & physical health
11. Tepas [36] Shift work Sequential Austrian life span model from Kundi et al. (1979) and Haider et al. (1981). Development of health effects occurs in 4 sequential phases: adaptation phase (0–5 years), sensitisation phase (5–20 years), accumulation phase (20+ years), disease manifestation phase (40+ years). Situational and biological factors may have a stabilising or destabilising effect on each phase. Shift work drop-out may occur at each stage depending on individual stress tolerance and coping development. Disease
12. Smith & Barton [54] Shift work Appraisals of controllability (shift work locus of control) together with actual control over working hours influence situational control, which in turn plays a moderating role in the stress–strain process. External stress arises from working the shift system and internal strain from attempts to cope with the shift system. Reduced situational control may lead to negative health effects and absenteeism. Absenteeism
13. Barton [34] Shift work Same as Folkard [51].  
14. Richardson & Maly [33] Shift work Two theories were briefly described that may increase health risk: Shift work sleep disorder
 1. Chronic circadian disruption, i.e. chronic disruption of normative physiological processes.
 2. Chronic sleep deprivation, with proximal causes such as heightened exposure to behavioural risk factors.
15. Smith [44] Shift work Individual and situational variables influence the development of sleep, social, and domestic disturbances. The disturbances lead to coping behaviour (active or passive) to handle the stress associated with the disturbances. When coping is unsuccessful, it may lead to short-term effects (decreased emotional and physical well-being) which in turn may result in chronic health problems. Short-term: fatigue.
Chronic: digestive and cardiovascular symptoms
16. Perrucci [49] Shift work Schedule and job demands are stressors that predict negative health effects. Timing and duration of work and non-work are important: they may negatively (e.g. circadian disruption) or positively (e.g. more time off) influence health. The predictor variables are moderated or mediated by family and work place variables. Demographic and personality variables (e.g. shift work tolerance) independently influence predictor variables. Physical symptoms
Mental health and well-being
17. Reinberg [50] Shift work The Dian Circadian Model is described. Shift work and physical and/or psychological workload may lead to circadian disruption in some, but not in all individuals (euchronism). Circadian disruption may be present with clinical symptoms (dyschronism) or without clinical symptoms (allochronism). These inter-individual differences are due to genetic differences. Long-term exposure to circadian disruption may sensitise the body, leading from allochronism to dyschronism at a later stage. Clinical symptoms
18. Puttonen [21] Shift work Shift work may lead to circadian stress by disturbing circadian rhythms. Circadian stress is psychosocial stress (e.g. recovery), behavioural stress (e.g. sleep), and/or physiological stress (e.g. inflammation). These three stresses influence each other and may lead to other disease conditions (e.g. atherosclerosis) that precede cardiovascular disease. Physiological stress may directly lead to cardiovascular disease; so may other disease conditions. Cardiovascular disease
19. Antunes [42] Shift work The cause of obesity is a complex interplay between genetic, environmental, psychobehavioural, endocrine, and metabolic factors. Shift work leads to desynchronisation of work, social, and eating patterns, which may cause desynchronisation between central and peripheral oscillators. This in turn may cause weight gain by: Obesity
 1. Lower metabolic efficiency when eating at night due to gene expression at the ‘wrong’ time of day.
 2. Fat production in adipose tissue by increased sympathetic output due to stress, job strain, and psychosocial factors.
 3. Altered glucose and lipid homeostasis due to light at night.
 4. Lifestyle changes, e.g. reduced physical activity.
20. Fritschi [19] Shift work Shift work may lead to one or more of these mechanisms: Cancer
 1. Desynchronisation between central and peripheral oscillators causing physiological disruptions and intra-cellular disruptions.
 2. Light at night may suppress melatonin excretion and thereby reduce its anti-carcinogenic effects.
 3. Sleep disruption may cause stress axis activation and immune suppression.
 4. Lifestyle disturbances may lead to negative lifestyles and metabolic changes.
 5. Less sunshine for night workers may decrease production of Vitamin D and reduce its anti-carcinogenic effects.
21. Kivimäki [45] Shift work Shift work may cause desynchronisation between central and peripheral oscillators and trigger “a cascade of biological changes that have potential diabetogenic effects”. Shift work may also lead to poor or insufficient sleep. Desynchronisation between central and peripheral oscillators and poor or insufficient sleep affect each other, and may lead to insulin resistance and weight gain. This in turn may lead to diabetes type 2. Diabetes type 2
22. Dickerman & Lui [35] Night work Model of Fritschi et al. [20]. Review on evidence of light at night among female nurses working night shifts. Breast cancer
23. Vallières & Bastille-Denis [40] Night work Psychobiological model. Night work may disrupt sleep regulation that leads to shift work sleep disorder. Sleep regulation is controlled by a complex interaction between circadian rhythm and sleep homeostasis, in which adjustment capacity (plasticity) and involuntary processes (automaticity) play a central role. Subsystems protect plasticity and automaticity; these include physiological (e.g. chronotype) and cognitive de-arousal (e.g. no intrusive thoughts), as well as stimulus control (e.g. sleep habits) and facilitation of daytime sleep (e.g. low job stress). Shift work sleep disorder
24. Haines [41] Extended working hours Extended working hours are not a stressor; rather, they are a parameter for duration of physical or mental effort. Therefore, the association between extended working hours and psychological distress is mediated by increased psychological work demands and increased decision latitude. Psychological distress
25. Caruso [22] Nonstandard working schedules The paper describes the Framework for Study of Undesirable Impacts of Long Work. Extended working hours, together with other schedule characteristics, lead to longer exposure to job demands, and reduce the time for recovery and sleep. This may lead to acute effects and chronic illnesses. Worker and job characteristics moderate the associations between time for recovery and acute effects, and between acute effects and chronic illnesses. Chronic illnesses may increase vulnerability to acute effects. Acute effects and Chronic illnesses
26. Steinmetz & Schmidt [32] Nonstandard working schedules 1. Sequence model: The effect of job stressors and working time on health outcomes is mediated by sleep quality and chronic fatigue. 1. Gastro-intestinal, cardio-vascular and musculo-skeletal complaints
2. General strain factor model: The effect of job stressors and working time on health is mediated by a general strain factor. A general strain factor is a common factor underlying the simultaneous expression of various health constructs and it is explained by the process of sensitization of cognitive, emotional, and somatic systems. This model is a better fit to the study data than the sequence model. 2. Somatic complaints, chronic fatigue, sleep quality