Chronobiology international
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Randomized Controlled Trial Clinical Trial
The effect of time-of-day and sympathetic α1-blockade on orthostatic tolerance.
Tolerance time to a standardized orthostatic stressor is markedly reduced in normotensive individuals in the morning. However, the physiological mechanisms that underpin this phenomenon are unknown. The purpose of this study was to examine the role of α(1)-adrenergic activity on orthostatic tolerance and associated cardiorespiratory and cerebrovascular responses, and to determine whether its endogenous modulation is important in the diurnal variation of orthostatic tolerance. ⋯ Independent of time-of-day, α(1)-blockade markedly reduced the ability to tolerate a 15-min 60° HUT; tolerance time was 229% shorter compared with the placebo condition (p ≤ .0001). Moreover, a marked diurnal variation in orthostatic tolerance was evident following α(1)-adrenergic blockade; e.g., tolerance time in the morning (176 ± 30 s) was lower than in the afternoon (354 ± 75 s; p = .04). These findings highlight an important role of α(1)-sympathetic vasoconstrictor activity in acutely regulating blood pressure and offsetting syncope, especially in the early morning.
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The "Bergen Shift Work Sleep Questionnaire" (BSWSQ) was developed to systematically assess discrete sleep problems related to different work shifts (day, evening, night shifts) and rest days. In this study, we assessed the psychometric properties of the BSWSQ using a sample of 760 nurses, all working in a three-shift rotation schedule: day, evening, and night shifts. BSWSQ measures insomnia symptoms using seven questions: >30-min sleep onset latency, >30-min wake after sleep onset, >30-min premature awakenings, nonrestorative sleep, being tired/sleepy at work, during free time on work days, and when not working/on vacation. ⋯ Night shift showed the highest score compared to day and evening shifts as well as to rest days (all post hoc comparisons p < .001). Mean scores of different symptoms also varied significantly within the individual work shifts. We conclude that the BSWSQ meets the necessary psychometric standards, enabling systematic study of discrete insomnia symptoms in different work shifts.
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Most night workers are unable to adjust their circadian rhythms to the atypical hours of sleep and wake. Between 10% and 30% of shiftworkers report symptoms of excessive sleepiness and/or insomnia consistent with a diagnosis of shift work disorder (SWD). Difficulties in attaining appropriate shifts in circadian phase, in response to night work, may explain why some individuals develop SWD. ⋯ Finally, SWD subjects were exposed to more morning light (between 05:00 and 11:00 h) as than ANW ones (798 vs. 180 lux [M ± SD], respectively z = -1.7; p < .05). These data provide evidence of an internal physiological delay of the circadian pacemaker in asymptomatic night-shift workers. In contrast, individuals with SWD maintain a circadian phase position similar to day workers, leading to a mismatch/conflict between their endogenous rhythms and their sleep-wake schedule.
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Previous studies have shown increased sleepiness and mood changes in shiftworkers, which may be due to sleep deprivation or circadian disruption. Few studies, however, have compared responses of experienced shiftworkers and non-shiftworkers to sleep deprivation in an identical laboratory setting. The aim of this laboratory study, therefore, was to compare long-term shiftworkers and non-shiftworkers and to investigate the effects of one night of total sleep deprivation (30.5 h of continuous wakefulness) and recovery sleep on psychomotor vigilance, self-rated alertness, and mood. ⋯ As expected, both groups showed a decrease in subjective alertness and PVT performance during and following the sleep deprivation night. Subjective sleepiness and most aspects of PVT performance returned to baseline levels after a nap and recovery sleep. The mechanisms underlying the observed differences between shiftworkers and non-shiftworkers require further study, but may be related to the absence of shiftwork the week prior to and during the laboratory study as well as selection into and out of shiftwork.