Alexandra L. Holmes

Bio: Alexandra L. Holmes is an academic researcher from University of South Australia. The author has contributed to research in topics: Sleep debt & Circadian rhythm. The author has an hindex of 8, co-authored 13 publications receiving 356 citations.

The impact of a week of simulated night work on sleep, circadian phase, and performance.

Abstract: Aims: To investigate factors that may contribute to performance adaptation during permanent night work. Methods: Fifteen healthy subjects participated in an adaptation and baseline night sleep, directly followed by seven simulated eight-hour night shifts (2300 to 0700 hours). At the end of each shift they were taken outside and exposed to natural light for 20 minutes. They then slept from approximately 0800 hours until they naturally awoke. Results: There was a significant increase in mean performance on a visual psychomotor vigilance task across the week. Daytime sleep quality and quantity were not negatively affected. Total sleep time (TST) for each of the daytime sleeps was reduced, resulting in an average cumulative sleep debt of 3.53 hours prior to the final night shift. TST for each of the daytime sleep periods did not significantly differ from the baseline night, nor did TST significantly vary across the week. There was a significant decrease in wake time after sleep onset and sleep onset latency across the week; sleep efficiency showed a trend towards greater efficiency across the consecutive daytime sleeps. Hours of wakefulness prior to each simulated night shift significantly varied across the week. The melatonin profile significantly shifted across the week. Conclusions: Results suggest that under optimal conditions, the sleep debt that accumulates during consecutive night shifts is relatively small and does not exacerbate decrements in night-time performance resulting from other factors. When sleep loss is minimised, adaptation of performance during consecutive night shifts can occur in conjunction with circadian adaptation.

The ability to self-monitor performance during a week of simulated night shifts.

Abstract: STUDY OBJECTIVES: Research has indicated that individuals are able to accurately monitor the performance decrements they experience during unitary periods of acute sleep deprivation. The aim of the current study was to investigate the ability to self-monitor performance during a week of simulated night shifts. DESIGN: Subjects completed 7 consecutive 8-hour night shifts (11 pm-7 am). SETTING: University sleep laboratory. SUBJECTS: Fifteen young (7 men, 8 women, 19-25 years) healthy volunteers. INTERVENTIONS: During the night shifts, performance was measured hourly on 4 performance parameters: psychomotor vigilance test (PVT), tracking, and grammatical reasoning (GRG) accuracy and response latency. Before and after each test, subjects completed visual analogue scales, which required them to rate their alertness and their performance speed and/or accuracy. RESULTS: Analysis indicated that GRG response latency and tracking were significantly impaired (P<0.05) during the first 2 shifts only. The PVT performance displayed consistent impairment, with significant (P<0.05) declines during all but the final shift. The pattern of deterioration in subjective ratings of alertness was similar to that of the PVT data. Correlations between subjective alertness and self-ratings of performance were significant (P<0.01) for all parameters (r=0.39-0.69). Significant (P<0.05) correlations were found across the week between pretest performance ratings and actual performance for all parameters except GRG accuracy (r=0.29-0.58) and between posttest ratings and actual performance for all parameters (r=0.52-0.75). Correlations between pretest ratings and actual performance were also conducted separately for each shift. Highest correlations were found during the first shift, with r-values that were low for GRG accuracy (r=0.32) and GRG response latency (r=0.20), moderate for tracking (r=0.41), and high for PVT (r=0.82). In general, lower correlations were found later in the week. CONCLUSIONS: Overall, results indicate that individuals have only a moderate ability to predict performance impairment during a week of night shifts. It is likely that performance ratings are based, at least to a certain extent, on subjective alertness levels. Furthermore, it seems that rating accuracy is improved on tasks providing performance feedback, such as the PVT. Finally, it appears that after testing, individuals have a more accurate perception of their performance. Language: en

Effects of sleep pressure on endogenous cardiac autonomic activity and body temperature

Abstract: This study investigated the effects of variations in sleep pressure on cardiac autonomic activity and body temperature. In a counterbalanced design, 12 healthy, young subjects (6 men and 6 women) r...

The relationship between slow-wave activity, body temperature, and cardiac activity during nighttime sleep.

Abstract: Study Objectives: Recent work indicates that cardiac sympathetic activity is not influenced by the circadian system and instead decreases after sleep onset. However, little is known about the pattern of change in cardiac sympathetic activity during NREM/REM sleep cycles and whether this is associated with alterations in slow-wave activity (SWA). To address these questions, we examined SWA, cardiac sympathetic activity, heart rate and rectal and foot temperatures during the first three NREM/REM sleep cycles and during transitions between NREM and REM sleep. Design: Subjects were required to maintain a constant sleep-wake cycle for at least a week and have at least one adaptation night, before their night of recording. Setting: Individual temperature controlled bedrooms. Participants: 10 young healthy males and females. Interventions: NA. Measurements and Results: All variables showed the greatest change in the first NREM cycle. Specifically, SWA, sympathetic activity, heart rate and foot temperature increased while rectal temperature decreased. After the initial increase, cardiac sympathetic activity decreased across the sleep phase, in association with a decrease in heart rate. Cardiac sympathetic activity did not significantly alter across NREM-REM cycles. Conclusions: The results suggest that increases in heart rate and cardiac sympathetic activity early in the sleep period are, in part, a compensatory reaction to the concomitant thermoregulatory changes observed. These results also indicate that the effect of time asleep on cardiac sympathetic activity may be greater than the influence of sleep cycles. These results are discussed with reference to the recuperative value of naps.

Daytime cardiac autonomic activity during one week of continuous night shift.

Abstract: Shift workers encounter an increased risk of cardiovascular disease compared to their day working counterparts. To explore this phenomenon, the effects of one week of simulated night shift on cardiac sympathetic (SNS) and parasympathetic (PNS) activity were assessed. Ten (5m; 5f) healthy subjects aged 18-29 years attended an adaptation and baseline night before commencing one week of night shift (2300-0700 h). Sleep was recorded using a standard polysomnogram and circadian phase was tracked using salivary melatonin data. During sleep, heart rate (HR), cardiac PNS activity (RMSSD) and cardiac SNS activity (pre-ejection period) were recorded. Night shift did not influence seep quality, but reduced sleep duration by a mean of 52 +/- 29 min. One week of night shift evoked a small chronic sleep debt of 5 h 14 +/- 56 min and a cumulative circadian phase delay of 5 h +/- 14 min. Night shift had no significant effect on mean HR, but mean cardiac SNS activity during sleep was consistently higher and mean cardiac PNS activity during sleep declined gradually across the week. These results suggest that shiftwork has direct and unfavourable effects on cardiac autonomic activity and that this might be one mechanism via which shiftwork increases the risk of cardiovascular disease. It is postulated that sleep loss could be one mediator of the association between shiftwork and cardiovascular health.

Systematic interindividual differences in Neurobehavioral impairment from sleep loss: Evidence of trait-like differential vulnerability

Abstract: OBJECTIVES: To investigate interindividual differences in neurobehavioral deficits during sleep deprivation, and to establish to what extent the neurobehavioral responses to sleep loss are a function of sleep history versus trait-like differential vulnerability. DESIGN: Individuals were exposed to sleep deprivation on 3 separate occasions in order to determine the stability of interindividual differences in neurobehavioral impairment. SETTING: The sleep-deprivation experiments were conducted under standardized laboratory conditions with continuous monitoring of wakefulness. Each subject underwent a laboratory-adaptation session before entering the sleep-deprivation phase of the study. PARTICIPANTS: A total of 21 healthy adults (aged 21-38 years) completed the experiment. INTERVENTIONS: Subjects came to the laboratory 3 times at intervals of at least 2 weeks. During each laboratory session, they underwent neurobehavioral testing every 2 hours during 36 hours of total sleep deprivation, which was preceded by baseline sleep and followed by recovery sleep. In the week prior to each sleep-deprivation session and on the baseline night in the laboratory, subjects were required to either restrict their sleep to 6 hours per day (prior sleep restriction condition) or to extend their time in bed to 12 hours per day (prior sleep extension condition), so as to experimentally manipulate sleep history (in randomized counterbalanced order). RESULTS: There was strong evidence that interindividual differences in neurobehavioral deficits during sleep deprivation were systematic and trait-like. The magnitude of interindividual variability was substantial relative to the magnitude of the effect of prior sleep restriction (which on average involved a reduction of 4.1 hours sleep per day, compared to prior sleep extension, for 7 days). Overall, interindividual differences were not explained by subjects' baseline functioning or a variety of other potential predictors. Interindividual variability clustered on 3 distinct neurobehavioral dimensions: self-evaluation of sleepiness, fatigue, and mood; cognitive processing capability; and behavioral alertness as measured by sustained attention performance. CONCLUSIONS: Neurobehavioral deficits from sleep loss varied significantly among individuals and were stable within individuals. Interindividual differences in neurobehavioral responses to sleep deprivation were not merely a consequence of variations in sleep history. Rather, they involved trait-like differential vulnerability to impairment from sleep loss, for which neurobiologic correlates have yet to be discovered. Language: en

Systematic Interindividual Differences in Neurobehavioral Impairment from Sleep Loss: Evidence of Trait-Like Differential Vulnerability SLEEP, SLEEP RESTRICTION, AND PERFORMANCE

Abstract: OBJECTIVES To investigate interindividual differences in neurobehavioral deficits during sleep deprivation, and to establish to what extent the neurobehavioral responses to sleep loss are a function of sleep history versus trait-like differential vulnerability. DESIGN Individuals were exposed to sleep deprivation on 3 separate occasions in order to determine the stability of interindividual differences in neurobehavioral impairment. SETTING The sleep-deprivation experiments were conducted under standardized laboratory conditions with continuous monitoring of wakefulness. Each subject underwent a laboratory-adaptation session before entering the sleep-deprivation phase of the study. PARTICIPANTS A total of 21 healthy adults (aged 21-38 years) completed the experiment. INTERVENTIONS Subjects came to the laboratory 3 times at intervals of at least 2 weeks. During each laboratory session, they underwent neurobehavioral testing every 2 hours during 36 hours of total sleep deprivation, which was preceded by baseline sleep and followed by recovery sleep. In the week prior to each sleep-deprivation session and on the baseline night in the laboratory, subjects were required to either restrict their sleep to 6 hours per day (prior sleep restriction condition) or to extend their time in bed to 12 hours per day (prior sleep extension condition), so as to experimentally manipulate sleep history (in randomized counterbalanced order). RESULTS There was strong evidence that interindividual differences in neurobehavioral deficits during sleep deprivation were systematic and trait-like. The magnitude of interindividual variability was substantial relative to the magnitude of the effect of prior sleep restriction (which on average involved a reduction of 4.1 hours sleep per day, compared to prior sleep extension, for 7 days). Overall, interindividual differences were not explained by subjects' baseline functioning or a variety of other potential predictors. Interindividual variability clustered on 3 distinct neurobehavioral dimensions: self-evaluation of sleepiness, fatigue, and mood; cognitive processing capability; and behavioral alertness as measured by sustained attention performance. CONCLUSIONS Neurobehavioral deficits from sleep loss varied significantly among individuals and were stable within individuals. Interindividual differences in neurobehavioral responses to sleep deprivation were not merely a consequence of variations in sleep history. Rather, they involved trait-like differential vulnerability to impairment from sleep loss, for which neurobiologic correlates have yet to be discovered.