Introduction

Shift work does not have a specific definition; whatever work we perform apart from our regular day-to-day working hours is considered a shift work. Shift work, being performed in ~22% of population in industrialized countries is an essential part of numerous industries.1 2 Various forms of illnesses including cardiovascular diseases, diabetes, and obstructive sleep apnea are the consequences of night shift work.3 Diseases such as hypertension, left ventricular hypertrophy, coronary heart disease, and myocardial infarction are very common and could be critical in night shift workers than in day shift workers.2 All these complications arise as an outcome of the encounter that appears between shifted work hours and the output of the biological clock.4 The ability to be synchronized (or entrained) by zeitgebers (external time cues) is a noteworthy feature of circadian rhythm.5 For synchronizing endogenous rhythms, light is the most effective stimulus. The time, duration, intensity, and the wavelength of light cause synchronization6 via the retinohypothalamic tract to the supra chiasmatic nucleus (SCN) photic information is transmitted in mammals.7 Stimuli including scheduled voluntary exercise; food; and exogenous melatonin or serotonergic activation, that is, nonphotic stimuli can additionally shift the endogenous circadian rhythms.

Nocturnal feeding disturbs the synchronization of circadian rhythms and has been shown as a strong entrainer.8 A sudden change in the feeding time schedule (from day to night) for some days progressively shifts the phases of the peripheral clocks; it may occur as a result of night shift working hours that directly disrupt circadian rhythm.9 When suitably timed, exogenous melatonin shifts the phase of the human circadian clock to the earlier (advance phase shift) or later (delay phase shift) times, having acute sleepiness-inducing and temperature-lowering effects during the biological daytime.10

After an unexpected change in an individual’s habits and/or environment, a momentary nap throughout the day or a temporary nocturnal awakening that does not trigger a change in the body’s endogenous clock may lead to the resynchronization of rhythms. A disparity between the endogenous and exogenous components of the rhythm appears when the difference occurs over a long period (~5 days).11 Poor health is directly linked with this desynchronization of circadian rhythm. Sleep disturbances, shift-lag syndrome, increased risk of errors, and work-related accidents are the short-term consequences of such desynchronization whereas the long-term consequences lead in disturbances of the cardiovascular systems, reproductive difficulties, impaired lipid metabolism, and cancer.12 Here in this study, we have demonstrated that circadian misalignment increases blood pressure and may affect inflammatory markers. Our findings may help explain why shift working nurses have increased hypertension and cardiovascular disease risk. Therefore, in this study we have measured the disruption in circadian rhythm in night shift working nurses (NSWN).

Materials and Methods

Questionnaires: General Assessment Questionnaire Was Analyzed

Results

We examined 25 day shift working nurses (DSWN) with mean age 36.7 ± 9.9 years and 25 night shift working nurses (NSWN) with mean age of 32.8 ± 10.5 years from ELMCH. The 7-day/24-hour blood pressure monitoring recorded in nurses consisted of two groups with day working shifts and night working shift, respectively. During the monitoring, we evaluated the sleep time on different days in every nurse by the sleep questionnaire. Both the group of nurses, the DSWN and the NSWN, were recruited for 7-day ABPM. Medical instruments TM2430 (A&D, Japan) monitors were used for ABPM (oscillation method). Means of systolic and diastolic BP were evaluated. We calculated mean systolic and diastolic BP for 7 days and for every 24-hour profile. Blood was withdrawn from both the group of nurses after ABPM. Consent was obtained and the ethics committee of our institution gave approval for the study. General assessment questionnaire was filled which contained personal identification variables. The casual BP measurement was performed three consecutive times with a validated automatic device with a 2-minute interval between the measurements, with appropriate cuff to the arm size, in the seated position. Presence of at least one of the following possibilities characterized hypertension: usage of antihypertensive drugs, self-reported hypertension, or the values of the spontaneous measure greater than or equal to 140 mm Hg and/or 90 mm Hg.

ABPM was executed for 24 hours. Based on the schedules described in the participants’ activity reports the averages and prevalence of 24-hour hypertension, awake and sleep periods were evaluated. The accepted cutoff points for hypertension for systolic and diastolic pressures, respectively, were ≥130/80 mm Hg in the 24-hour period, ≥135/85 mm Hg in wakefulness, and ≥120/70 mm Hg in the sleep period Fig. 1.

Discussion

Shift work is any work that exceeds more than 8 hours of regular working period, therefore interfering with the natural cycle of biological rhythm of our body. Workers in night shift have a greater risk for CVD in comparison to an individual working normal hours; night time shift working is more hazardous to health as it directly impacts the biological and physiological system of the body. However, the exact mechanism of the impact of shift work on cardiovascular health is yet to be understood. BP modifications may be a possible factor in that nocturnal work modifies the circadian rhythm of BP and those individuals in whom there is a fall in nocturnal BP (dippers) become nondippers when they start working at night time. In a single day of working in night hours there is a high systolic BP level.13

In this article, we examined 25 DSWN with mean age 36.7 ± 9.9 years and 25 NSWN with mean age 32.8 ± 10.5 years. The 7-day/24-hour BP monitoring recorded in nurses consisted of two groups with DSWN and NSWN. During the monitoring, we evaluated the sleep time on different days in every nurse using the sleep questionnaire. Both groups of nurses, the DSWN and the NSWN, were recruited for 7-day ABPM.

An integral part in sleeping time differences between the NSWN and DSWN was also observed by the questionnaire. As in NSWN there is already a lack of rest activity that is very important for our health and not resting properly directly elevates catecholamine, resulting in interference in normal BP. NSWN were not different from the DSWN. Maybe because the similar loads in BP can represent different mean readings of BP, we, therefore, could not find any difference in that in our study. More than 53 mm Hg pulse pressure is also related to a greater risk of CVD.14 Perhaps, because we had more young and healthy individuals in our study, no difference was found in the pulse pressure readings in both the groups.

In NSWN, the mean BP was higher, although in statistics the difference was recorded only in the systolic and diastolic 24-hour BP and the diastolic BP. Our findings are in agreement with other publications.15 Moreover, a study on nurses working in night found lower systolic BP and average readings of diastolic BP. However, in that study, the comparison was done on the basis of daytime reading of BP for day workers and nighttime reading of BP for night workers.16

During the 24 hours of night shift work more frequent high abnormal mean reading of BP was observed. This study is very similar to a study that was conducted on 62 doctors for evaluating BP with an ambulatory blood pressure monitor. In that study it was observed that only 3 patients in the 24-hour period had abnormally high mean BP readings, whereas 6 and 9 patients during the work period had high mean systolic BP and diastolic BP readings.17 It was suggested through study that such cases could resemble “masked hypertension,” that is, a condition where the BP levels are normal in office but rise when measured with ambulatory blood pressure monitor.18

A study by Bevan et al showed 16 junior medical staff in a 24-hour shift work in the emergency room and in common working day and found higher nocturnal systolic BP and diastolic BP readings. It also found lower nocturnal BP fall during the 24-hour night shift work.19 Another study with 37 residents reported higher BP loads and less night time BP fall during 24-hour shift work.20 In contrast to our study, these studies used a fixed time period to describe sleep and awake periods without corrections for the real period of wakefulness and sleep, which probably led to higher nocturnal BP readings and consequently to less nocturnal BP fall. These results also found higher mean BP readings during the 24-hour shift work, including values considered as hypertensive in few or some periods.

During the last few years, many studies have been published dealing in different works of many genetics and biochemical parameters studied in cardiovascular diseases. This study is, however, the most extensive so far to look into the relation between the NSWN and DSWN and cardiovascular alterations, mainly BP levels in young nurses in critical care unit, by analyzing ABPM and the circadian rhythm questionnaire. Further, in this study we observed that nurses working in night shifts in the critical care have higher mean 24-hour systolic, diastolic, and sleeping time diastolic BP levels compared with those working during the day time. This may happen because of lack of sleep, and lack of sleep directly elevated mental stress. Moreover, mean BP levels above the normal values, especially the 24-hour diastolic BP and BP levels in all time periods, were related to 24-hour shift work in the critical care.

Conclusion

The 7-day/24-hour ABPM in NSWN showed reduced circadian rhythm variations in BP as compared with DSWN. Work schedules and the consequent rest–activity schedules affect circadian rhythms, with likely long-term impact on health. Therefore, working in the critical care unit during the night hours leads to abnormal BP in NSWN, suggesting this type of work, in which sleep is disturbed leading to mental stress, could be a direct risk factor in cardiovascular diseases.

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Fig. 1 Disruption in circadian rhythm causes increased risk of cardiovascular diseases in night shift working nurses.

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Fig. 1 Disruption in circadian rhythm causes increased risk of cardiovascular diseases in night shift working nurses.