Police officers suffer disproportionately from a variety of adverse health outcomes, including sleep disruption, psychological stress, depression, suicide, metabolic disorders, heart disease, diabetes, and higher mortality (Franke et al., 1997; Vena et al., 1986; Vila, 2006; Violanti et al., 1998, 2006). Increased risks for developing heart disease among police likely arise, at least in part, from a higher prevalence of hypertension and obesity in this occupational group compared to the general population (Franke et al., 1997; Ramey et al., 2008). In addition, higher rates of cancer, including those of the colon, bladder, digestive tract, brain, esophagus, hematopoietic tissues, as well as lymphomas and melanomas have been associated with police work (Forastiere et al., 1994; Vena et al., 1986). The lengthy, stressful, and sometimes life-threatening work that officers face often requires working irregular hours, including nights. There is considerable overlap between diseases observed among police officers and those observed in other occupations that include shiftwork. For example, shiftwork has been associated with higher risks of prostate, breast, and colorectal cancer, non-Hodgkin's lymphoma, elevated blood pressure, and circulating cholesterol, cardiovascular disease (CVD), and increased rates of occupational accidents and injuries (Boggild & Knutsson, 1999; Chen et al., 2010; Conlon et al., 2007; Dembe et al., 2006; Ha & Park, 2005; Horwitz & McCall, 2004; Karlsson et al., 2003; Kubo et al., 2006; Lavie & Lavie, 2007; Schernhammer et al., 2001, 2003). However, the extent to which excesses in morbidity and mortality among police officers are due to shiftwork is not completely understood, and research on this subject has been limited.

Shiftwork can result in circadian dysregulation, sleep insufficiency, and disruption of familial and social life (Åkerstedt, 1990, 1998; Burch et al., 2005,2009; Camerino et al., 2010; Gordon et al., 1986; Harada et al., 2005; Ohayon et al., 2010; Shields, 2002), which can lead to increased allostatic load (defined as strain on the body and changes in metabolism produced by repeated cycling of the stress response) (McEwen & Stellar, 1993), altered homeostasis, plus immune, endocrine, and autonomic dysfunction (Navara & Nelson, 2007). These changes may contribute to the pathophysiological mechanisms underlying the elevated disease risks that have been observed among shiftworkers. Psychological impacts, in some instances manifested as increases in depressive symptoms, have also been observed among shiftworkers through use of the Center for Epidemiologic Studies Depression (CES-D) scale or other instruments that ascertain mood (Cavallo et al., 2002; Coffey et al., 1988; Gordon et al., 1986; Harada et al., 2005; Takahashi et al., 2006). The specific biological processes that are linked with these adverse outcomes have not been fully characterized, but are likely to be critical to the development of effective disease prevention and intervention strategies among police officers, as are the temporal aspects of shiftwork-related physiological or psychological perturbations.

Cortisol is an adrenal steroid classically referred to as a “stress hormone” (Henry, 1992) that can be conveniently measured in saliva where its secretion pattern mimics concentrations in circulation (Lippi et al., 2009). Acute exposure to a stressor can result in elevated circulating cortisol levels for periods of 45 to 60 min after initiation of the stress response (Clow et al., 1997; Khalfa et al., 2003). There are a number of regulatory processes that facilitate maintenance of basal cortisol levels after initiation of the stress response (McEwen, 2006; McEwen & Stellar, 1993). However, during prolonged periods of stress, the ability of the hypothalamic-pituitary-adrenal (HPA) axis to self-regulate cortisol secretion may become compromised, which can result in an “exhaustion stage” or inability of the body to regulate the stress response. This breakdown in control of the stress response, sometimes referred to as allostatic overload (McEwen, 2008), can be manifested as a change in the pattern of the awakening response or a failure of the body to control the release of cortisol after stimulation (Motzer & Hertig, 2004). The cortisol awakening response (CAR) is characterized by a 40–75% increase of salivary cortisol levels, with maximum values occurring about 30 min after awakening (de Weerth et al., 2003; Fries et al., 2009; Kudielka et al., 2006). The CAR can be characterized via collection of saliva samples within the first hour of awakening and computation of the area under the curve (AUC) for the salivary cortisol profile (Fekedulegn et al., 2007; Pruessner et al., 1997; Wust et al., 2000). The CAR is a discrete and dynamic component of daily cortisol secretion that is heritable and remains stable within individuals over at least several days under normal circumstances (Clow et al., 2004; Pruessner et al., 1997; Wust et al., 2000). A recent meta-analysis of 62 studies concluded that job stress and overall life stress were positively associated with waking cortisol AUC measures (Chida & Steptoe, 2009). In contrast, decreases in waking cortisol measures have been observed in populations where chronic stress might be expected, including depressed individuals and among police officers with symptoms of posttraumatic stress disorder (PTSD) (Huber et al., 2006; Neylan et al., 2005). Changes in cortisol secretion in response to various stressors have been linked with perturbations in physiological processes that facilitate inflammation, and with other risk factors for CVD, type II diabetes, stroke, and cancer (Gidron & Ronson, 2008; Goh et al., 2000; Scheer et al., 2008; Zuzewicz et al., 2000). This indicates that salivary cortisol biomonitoring may be useful for characterizing the pathophysiological effects of stress and shiftwork, as suggested by a recent study (Oginska et al., 2010).

The Buffalo Cardio-Metabolic Occupational Police Stress (BCOPS) cohort is a prospective study that is examining biological processes by which the stressors associated with police work may mediate adverse health outcomes. The BCOPS protocol has combined the characterization of stress biomarkers, including the CAR, with subclinical CVD measures and psychosocial factors to examine their potential associations with irregular work hours, psychological disturbances, and chronic diseases afflicting police officers (Charles et al., 2007; Violanti et al., 2006, 2007, 2008, 2009a, 2009b). The objective of the current analysis was to evaluate the effect of long- and short-term shiftwork on the CAR and depressive symptoms among officers participating in the BCOPS pilot study, and to estimate the extent of shiftwork required to exert impacts on these endpoints. Specifically, this study tested the hypotheses that officers working predominantly night or afternoon shifts had altered salivary waking cortisol AUC measures and increased depressive symptoms compared to those working days. It is still uncertain whether night work per se, more frequent shift changes, or extended work hours may be more detrimental to long-term worker health (Crowley et al., 2004; Quera-Salva et al., 1997; Reinberg & Ashkenazi, 2008; Reinberg et al., 2007). For this reason, we used several different summary measures, including the cumulative number of work hours, or cumulative shift changes experienced by each officer, as well as the cumulative amount of time on afternoon and/or night shifts, summarized over two different time frames, to examine these possibilities. The short-term shiftwork period summarized the work schedule between 3 and 14 days prior to participation and sample collection, and the long-term shiftwork period summarized each subject's cumulative irregular work hours from 1994 until their participation in the study in 2001.

Complete data were available for 65 participants with long-term shiftwork data (n = 32 missing one or more of four waking cortisol samples, n = 3 missing long-term shiftwork data) and 58 subjects with short-term shiftwork data (n = 32 missing cortisol, n=10 missing short-term shiftwork data). Characteristics of officers grouped according to their long-term shift status (days, afternoons, or nights) are presented in Table 1. The average duration of police work (± standard deviation [SD]) in this population was 14 ± 9 yrs with a range of 1 to 33 yrs. The mean age was 43 ± 8 yrs with a range of 29 to 63 yrs. Males comprised 65% of the study participants. European Americans (EAs) comprised 71% of the study population, and 29% were African American (AA). Those working mainly afternoon or night shifts tended to be younger, male, EA, and had lower IES scores and fewer years of service compared to those working day shifts. A greater proportion of participants working night shifts held the rank of police officer (100%) compared to those on day shifts (47%). On average, participants classified as afternoon or night workers spent >85% of their time working irregular (night or afternoon) shifts (Table 1). Long-term cumulative night work was modestly correlated with night work summarized over the preceding 5 or 14 days (r = 0.39, p ≤ .01 for both) (Table 2) and long-term afternoon work was correlated with afternoon work over the prior 5 or 14 days (r = 0.63 to 0.52, both p ≤ .01) (Table 2). Shift changes were correlated with long-term work on afternoon shifts (r = 0.64, p ≤ .01) but not with long-term night work, total work hours, or recent work on afternoon or night shifts (Table 2).

The salivary cortisol values for participating officers were generally representative of those found in other investigations, with higher levels in the morning and declining values over the day (Griefahn & Robens, 2008; Kudielka et al., 2007; Stone et al., 2001). Table 3 provides a summary of crude mean cortisol AUC_I and AUC_G, and CES-D values by population characteristic. Mean cortisol AUC_I values were elevated among females (229 vs. −6 among males, p ≤ .01), and there was a tendency for AUC_I values to be higher among those 40 to 49 yrs of age, although differences among age groups were not statistically significant. Cortisol AUC_G values tended to be elevated among those with a higher rank (captain/ detective: 955 vs. police officer: 679, p = .06). CES-D scores were not correlated with the cortisol AUC_I or AUC_G (both, r < 0.10, p > .45). Higher CES-D scores were observed among those with more education (>college: 5.2 vs. ≤high school: 2.9, p = .12), and elevated IES scores (>19) (8.1 vs. 3.2 among those with IES values ≤19, p<.01) (Table 3).

No statistically significant differences in adjusted-mean cortisol AUC_I or AUC_G were observed between officers classified as long-term afternoon, night, or combined shiftworkers compared to those working long-term day shifts (data not shown). Similarly, there were no statistically significant differences in adjusted-mean cortisol AUC_I or AUC_G between the lowest and highest quartiles of total work hours, or among quartiles of cumulative time on afternoon, night, or combined shifts (data not shown). There were no associations between continuous long-term shiftwork variables and the waking cortisol measures, nor were interaction terms between these variables and shift changes or with total work hours statistically significant (data not shown). Adjusted-mean cortisol AUC_I and AUC_G values were reduced among participants in the highest quartile of cumulative shift changes compared to those in the lowest quartile (AUC_I: −10 vs. 156, p = .07, and AUC_G: 568 vs. 826, p = .13, for upper vs. lower quartile, respectively). A statistically significant difference in the mean cortisol AUC_I was observed when these analyses were adjusted for time of awakening (AUC_I: −22 vs. 155, p = .05, and AUC_G: 786 vs. 517, p = .12, for upper vs. lower quartile, respectively).

When short-term shiftwork was evaluated, there was a statistically significant reduction in adjusted-mean AUC_G cortisol levels among officers working nights compared to those working day shifts over the preceding 3-, 5-, 7-, or 14-day periods (Table 4). Cortisol AUC_I was reduced in a manner similar as AUC_G, predominantly among those working nights or combined shifts, although none of those comparisons were statistically significant (Table 4). Figure 1 shows the percent difference in mean cortisol AUC_G between officers on irregular shifts compared to those working days. In all instances, mean cortisol AUC_G levels were greatest among day workers and lower among afternoon or night-shift workers, with a peak effect occurring when data were summarized over the 5 days immediately preceding study participation. In addition, we used repeated-measures analysis of variance with a spatial power covariance structure (to account for minor differences in data collection time points among subjects) to model cortisol concentrations as a function of collection time, shift, and the interaction between time and shift (5-day). The results from that analysis were consistent with the results described above; cortisol levels among night or afternoon workers were lower compared to those working days, and the time × shift interaction term was not statistically significant. There were no statistically significant differences in cortisol AUC_G or AUC_I between the upper and lower quartiles of total hours worked over 3, 5, 7, or 14 days, and the continuous forms of those variables were also not statistically significant. Furthermore, the interaction terms between irregular shift hours and total work hours summarized over the respective short-term work periods (3, 5, 7, or 14 days) did not suggest a combined effect among these variables (data not shown).

There were no statistically significant differences in adjusted-mean CES-D scores by shift status, between the upper and lower quartiles, or for the continuous forms of long-term afternoon, night, or combined shiftwork hours, total work hours, or shift changes (data not shown). There also were no statistically significant differences in adjusted-mean CES-D scores between those working primarily night shifts compared to those working days over the 3-, 5-, 7-, or 14-day periods, although there was a tendency for CES-D scores among night shiftworkers to be greater than day shiftworkers over 14 days (7.4 vs. 4.2 respectively, p = .14) (Table 4).

Diseases observed among police officers are similar to those seen in other occupations that include shiftworkers. However, the extent to which shiftwork or other types of occupational stressors contribute to the development of disease in this understudied, high-risk population remains uncertain. This study quantified shiftwork, total work hours, and shift changes over a period encompassing 972 person-years of police work in order to examine potential effects on the CAR and depressive symptoms. We observed shift-related reductions in the CAR compared to day workers. The largest differences occurred with cortisol AUC_G, but the patterns of response were similar for cortisol AUC_I. Cortisol secretion typically increases with the acute onset of different types of stressors (Kudielka et al., 2009) and with repeated challenges cortisol can remain elevated for extended periods with consequent flattening of the diurnal slope (McEwen, 2006). Thus, one might have expected CAR measures in this study to be elevated among shiftworking police officers. For example, in a study that compared different types of shift rotation, workers on a schedule that included three different rotating shifts over 7 days, with 5 subsequent days off, were compared with those on a schedule that included 3 working days on a given shift with 2 subsequent days off. Those on the 7-day rotating shifts had elevated diurnal cortisol AUCs (measured across the entire day) and a flattened diurnal cortisol slope compared to those with more frequent shift rotations (Lac & Chamoux, 2004). Other studies have observed reductions in waking as well as diurnal cortisol levels in response to shiftwork. For example, average waking cortisol concentrations were reduced by 44% after 2 wks of simulated night work (Griefahn & Robens, 2008). A reduction in mean waking salivary cortisol concentrations was reported among night workers after transitioning from daytime work in an electronics manufacturing facility (Kudielka et al., 2007). Among permanent nightworkers, waking salivary cortisol profiles were reduced both on night shifts and on days off (Kudielka et al., 2007).

In the present study, a maximal reduction in the CAR was observed when irregular work hours were averaged over 5 days, indicating that nearly a full week of afternoon or night shifts was required before maximal dysregulation of the CAR occurred. Typically, the officers repeatedly worked 4 days on, 3 days off, then 3 days on and 3 days off on a fixed shift. Thus, it is unlikely that officers routinely worked 5 continuous days during the week, but did so when they needed to cover a shift for a fellow employee. Other studies have observed changes in cortisol secretion patterns within a similar shiftwork time frame. For example, a reversal in salivary cortisol levels at 06:00 and 21:00 h was observed among nurses after 5 to 7 consecutive night shifts compared to those working days. After 7 days of night work, mean cortisol levels at 06:00 h decreased from about 16 nmol/L down to 4 nmol/L, whereas the 21:00 h cortisol values only increased from 3 to 8 nmol/L (Hennig et al., 1998). In another study, mean serum cortisol concentrations among night workers on a 3–4-day rotating schedule were elevated at the expected trough of circadian cortisol secretion (00:00 and 02:00 h) compared to controls (Touitou et al., 1990). There was a progressive increase in cortisol levels among controls from 02:00 to 08:00 h, but this pattern was not observed among the night workers, resulting in reduced cortisol amplitudes (Touitou et al., 1990). Changes in the rhythm or concentration of cortisol secretion have been associated with shiftwork in other studies (Goh et al., 2000; Leese et al., 1996; Munakata et al., 2001; Scheer et al., 2008; Zuzewicz et al., 2000), although not all results have been consistent (Axelsson et al., 2003; Boquet et al., 2004; Roden et al., 1993). For example, reductions in the waking cortisol AUC were recently reported among police officers with PTSD symptoms but not among night workers, although the type or duration of night work was unspecified (Neylan et al., 2005).

A strength of the current study is that shiftwork was objectively characterized by accessing payroll records and quantifying each participant's work history. The effects of irregular shifts varied by duration of shiftwork, and were independent of other potential confounding or modifying factors, including extended work hours and PTSD (IES) symptoms. Discrepancies between this and other studies may be due to certain uncharacterized factors, including differences in ambient light exposures among participants. Light exposure, particularly during the late dark phase or predawn hours, can activate cortisol production or alter the normal cortisol nadir (Clow et al., 2004; Sephton & Spiegel, 2003). Direct comparisons between our results and those reported by others also are difficult due to potential differences in the type of work performed, length, direction, or type of shift schedule that was used, time-of-day of sample collection or type of cortisol measure that was employed, or because of other differences, such as potential confounding factors that may not have been consistently characterized and evaluated among the published studies (Clow et al., 2004). One limitation of the present analysis is that some covariate information was not available (e.g., current tobacco, coffee, or alcohol consumption, and light exposure following awakening). In addition, saliva collection in the present study occurred on a training or off day, so officers working nights or afternoons had time to revert back to a typical diurnal schedule, which may have resulted in an attenuation of the effects we observed. In any case, our results were generally consistent with other studies indicating that waking cortisol levels or other aspects of cortisol secretion are disrupted in response to shiftwork (Goh et al., 2000; Griefahn & Robens, 2008; Kudielka et al., 2007; Leese et al., 1996; Munakata et al., 2001; Scheer et al., 2008; Touitou et al., 1990; Zuzewicz et al., 2000).

The shift change metric was the only long-term measure associated with the CAR, and the cortisol AUC_I was essentially ablated among officers in that group. Individuals in the highest quartile of shift changes had an average with SD of 105 ± 73 shift changes and an average work duration of 19 ± 9 yrs compared with an average of 5 ± 3 shift changes and 14 ± 9 yrs work duration among those in the lowest quartile. It is interesting to note that recent shiftwork over several days and frequent shift changes over periods of years both contributed to a reduction in the CAR. This diminution of cortisol secretion is more suggestive of the exhaustion stage of the stress response, when maladaptation and breakdown of the HPA's compensatory mechanisms are thought to occur due to frequent or excessive exposure to stressors (Motzer & Hertig, 2004). Reductions in the CAR have been linked with a variety of circumstances that may relate to adverse health, including advanced age (Kudielka & Kirschbaum, 2003), poor sleep quality among insomnia patients (Backhaus et al., 2004), chronic fatigue syndrome and symptoms of burnout (Pruessner et al., 1999; Roberts et al., 2004), PTSD (Rohleder et al., 2004), early loss experiences due to death or separation (Meinlschmidt & Heim, 2005), and depression (Stetler & Miller, 2005), as well as measures of adverse cardiovascular health (Hurwitz Eller et al., 2001) or reduced bone density in women (Brooke-Wavell et al., 2002; Clow et al., 2004). Continued research is needed to develop a better understanding of temporal relationships between stressor exposures, chronic dysregulation of the HPA axis and cortisol secretion, and associations with chronic disease risks among police officers or other shiftworking occupations, including evaluations of the intermediate time periods not included in this analysis.

Our study did not find that depressive symptoms were associated with cortisol AUC_I or AUC_G, which is not consistent with results from some previous studies. A recent meta-analysis found that depressed individuals had lower baseline cortisol levels than those without depression (Burke et al., 2005). Using the CES-D scale, elevated scores for depressive symptoms have been associated with shiftwork and increased sleepiness among rotating shiftworkers compared to those on permanent day shifts (Takahashi et al., 2006). A greater prevalence of depressive symptoms also was observed among pediatric residents working night float shifts (43%) when compared to working daytime rotations (14%) (Cavallo et al., 2002). In the present study, only about 6% of the population met the suggested definition of depression (CES-D scores ≥16), which is similar to what is expected in the general population (∼5%) (Weissman et al., 1996). It is possible that depressive symptoms were not frequent or severe enough in this population to detect shiftwork-related changes in cortisol AUC_G or AUC_I, or that coping measures or other psychological adaptation processes were effectively in place among the participants.

In summary, the BCOPS study is the first-ever population-based prospective cohort to characterize biomarkers of stress, subclinical CVD, body composition, and psychological symptoms in this high-risk occupation (Violanti et al., 2006). Measurement of the salivary CAR provides a valid and efficient procedure for biomonitoring of the HPA axis. The CAR exhibits a relatively a high degree of heritability, good intraindividual stability over several days (Bartels et al., 2003; Pruessner et al., 1997; Wust et al., 2000), and altered values have been consistently associated with psychometric measures of stress (Chida & Steptoe, 2009; Maina et al., 2009). Results from this investigation build upon our previous analyses showing a dysregulation of salivary cortisol secretion among officers with PTSD symptoms (Violanti et al., 2007). Disrupted sleep (Charles et al., 2007), increased suicide ideation (Violanti et al., 2008), and an elevated prevalence of metabolic syndrome (Violanti et al., 2009b) have also been observed among officers in the BCOPS cohort working irregular shifts. Our current results indicate that short-term night work and possibly long-term shift changes may lead to dysregulation of the CAR among police officers, which may help inform future analyses within the entire BCOPS cohort and in other populations that seek to determine whether changes in the CAR may be associated with the development of chronic disease.