***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: BACKGROUND The powers of the low-frequency ( LF ) and high-frequency ( HF ) oscillations characterizing heart rate variability ( HRV ) appear to reflect , in their reciprocal relationship , changes in the state of the sympathovagal balance occurring during numerous physiological and pathophysiological conditions . However , no adequate information is available on the quantitative resolution of this methodology . METHODS AND RESULTS We studied 22 healthy volunteers ( median age , 46.5 years ) who were subjected after a rest period to a series of passive head-up tilt steps r and omly chosen from the following angles : 15 degrees , 30 degrees , 45 degrees , 60 degrees , and 90 degrees . From the continuous ECG , after appropriate analog-to-digital conversion , a personal computer was used to compute , with an autoregressive methodology , time and frequency domain indexes of RR interval variability . Spectral and cross-spectral analysis with the simultaneously recorded respiratory signal excluded its contribution to LF . Age was significantly correlated to variance and to the absolute values in milliseconds squared of very-low-frequency ( VLF ) , LF , and HF components . The tilt angle was correlated to both LF and HF ( expressed in normalized units [ nu ] ) and to the LF-to-HF ratio ( r = .78 , -.72 , and .68 ; respectively ) . Lower levels of correlation were found with HF ( in ms2 ) and RR interval . No correlation was present between tilt angle and variance , VLF , or LF ( in ms2 ) . Individual analysis confirmed that the use of nu provided the greatest consistency of results . CONCLUSIONS Spectral analysis of HRV , using nu or LF-to-HF ratio , appears to be capable of providing a noninvasive quantitative evaluation of grade d changes in the state of the sympathovagal balance Background  Shift work  is associated with an increased rate of cardiovascular diseases and accidents . Discordance between circadian rhythms of stress-related biological variables and the work-sleep schedule might explain the reduced efficiency of work . It is not clear whether a shift schedule of work may induce similar discordance in the 24-hour oscillatory pattern of the cardiac autonomic control in respect to the work-sleep periods . Methods and Results Twenty-two healthy male blue-collar shift workers underwent 24-hour ECG recordings during each of the 3 different shifts (  first , 6 am to 2 pm  ; second , 2 to 10 pm ; third , 10 pm to 6 am ) . Spectral analysis of heart rate variability over 24 hours provided the normalized markers of cardiac sympathetic ( LFnu ) and vagal ( HFnu ) modulation of the sinoatrial node activity and of the sympathovagal balance ( LF/HF ) . LFnu and LF/HF exhibited 24-hour oscillations with different times of maximum and minimum in accordance with the working and sleeping periods , respectively . Lower values of LFnu and LF/HF suggestive of a reduced cardiac sympathetic modulation were present when the job task was performed at night compared with the values observed when the work was performed during morning and evening . Conclusions Continuous weekly changes of time of maximum and minimum in the cardiac sympathetic and vagal autonomic control may play a role in the excessive rate of cardiovascular diseases in shift workers . The reduced values of the indexes of cardiac sympathetic modulation during night work might be related to the presence of sleepiness or diminished alertness , which in turn could facilitate errors and accidents Background Low heart rate variability ( HRV ) is associated with a higher risk of death in patients with heart disease and in elderly subjects and with a higher incidence of coronary heart disease ( CHD ) in the general population . Methods and Results We studied the predictive value of HRV for CHD and death from several causes in a population study of 14 672 men and women without CHD , aged 45 to 65 , by using the case-cohort design . At baseline , in 1987 to 1989 , 2-minute rhythm strips were recorded . Time-domain  measures of HRV  were determined in a r and om sample of 900 subjects , for all subjects with incident CHD ( 395 subjects ) , and for all deaths ( 443 subjects ) that occurred through 1993 . Relative rates of incident CHD and cause-specific death in tertiles of HRV were computed with Poisson regression for the case-cohort design . Subjects with low HRV had an adverse cardiovascular risk profile and an elevated risk of incident CHD and death . The increased risk of death could not be attributed to a specific cause and could not be explained by other risk factors . Conclusions Low HRV was associated with increased risk of CHD and death from several causes . It is hypothesized that low HRV is a marker of less favorable health STUDY OBJECTIVES To study the adaptation and readaptation processes to 1 week of night work ( 6:30 PM to 6:30 AM ) followed by 1 week of day work ( 6:30 AM to 6:30 PM ) . DESIGN Part of a r and omized , placebo-controlled , crossover field study . Here , data from the placebo arm are presented . SETTING Oil rig in the North Sea . Work schedule : 2 weeks on a 12-hour shift , with the first week on the night shift and the second week on the day shift . PARTICIPANTS Subjects complaining about problems with adjusting to shift work . Seventeen workers completed the study . INTERVENTIONS N/A. MEASUREMENTS Subjective and objective measures of sleepiness ( Karolinska Sleepiness Scale and simple serial reaction time test ) and sleep ( diary and actigraphy ) . RESULTS Both subjective and objective measures improved gradually during night work . The return to day work after 1 week on the night shift led to a clear increase in subjective sleepiness and worsening of sleep parameters . During the week on the day shift , sleepiness and sleep gradually improved , similar to the improvement seen during night work . The workers indicated that the day shift was worse than the night shift on some of the measures , e.g. , sleep length was significantly longer during the night-shift period . CONCLUSIONS This is one of few studies showing how shift workers in a real-life setting adjust to night work . Both subjective and objective sleepiness and subjective sleep improved across days . The effects were especially pronounced for the subjective data Previously we have shown that the 6-sulphatoxymelatonin rhythm of oil rig workers on a 2-week night shift ( 1800 - 0600 h ) adapts to the shift via a phase delay . We now report the findings of a study on two offshore drill crews working a 1 week day ( 1200 - 0000 h ) , 1 week night ( 0000 - 1200 h ) swing shift . Urine sample s were collected every 2 - 3 h throughout the subjective days , with over-sleep collection s , for the measurement of 6-sulphatoxymelatonin by radioimmunoassay .  One crew ( n = 11 ) , studied in November , showed no change in their 6-sulphatoxymelatonin rhythm during night shift . The other crew ( n = 7 ) , studied in March , showed a significant phase advance of the rhythm during night shift . The data indicate that both the type of shift and the season influence the direction and degree of adaptation UNLABELLED Circadian Profile of Heart Rate Variability . INTRODUCTION Although heart rate variability ( HRV ) has been established as a tool to study cardiac autonomic activity , almost no data are available on the circadian patterns of HRV in healthy subjects aged 20 to 70 years . METHODS AND RESULTS We investigated 166 healthy volunteers ( 81 women and 85 men ; age 42 + /- 15 years , range 20 - 70 ) without evidence of cardiac disease .  Time-domain HRV parameters were determined from 24-hour Holter monitoring and calculated as hourly mean values and mean 24-hour values . All volunteers were fully mobile , awoke around 7 A.M. , and had 6 to 8 hours of sleep . Circadian profiles of vagus-associated HRV parameters revealed a marked day-night pattern , with a peak at nighttime and a plateau at daytime . The characteristic nocturnal peak and the day-night amplitude diminished with aging by decade . Estimates of overall HRV ( geometric triangular index [ TI ] , SD of NN intervals [ SDNN ] ) and long-term components of HRV ( SD of the averages of NN intervals for all 5-min segments [ SDANN ] ) were low at nighttime and increased in the morning hours . There was a significant decline of 24-hour values of all HRV parameters ( P < 0.001 ) and a strong negative correlation ( P < 0.001 ) with increasing age . Mean 24-hour RR interval ( P < 0.001 ) , SDNN , mean SD of NN intervals for all 5-minute intervals ( SDNNi ) , and SDANN ( all P < 0.01 ) were significantly higher in men . Younger men also exhibited significantly higher values for vagus-associated parameters ( root mean square successive difference [ rMSSD ] , P < 0.05 ; SDNNi , P < 0.01 ) ; however , gender differences diminished with increasing age . CONCLUSION Normal aging is associated with a constant decline of cardiac vagal modulation due to a significant decrease of nocturnal parasympathetic activity . The significant gender-related difference of HRV decreases with aging . These findings emphasize the need to determine age- , gender- , and nycthemeral-dependent normal ranges for HRV assessment Changes in sleep parameters during and after night-shift and the effects of bright white ( 2500 - 3000 1x ) and dim red ( > 500 1x ) light treatment on re adaptation after night-shift during winter were studied in 14 men on the British Anta rct ic Survey Base of Halley ( 75 degrees south ) . Subjects kept daily sleep diaries and mood ratings from one week before to three weeks after night-shift and received either full-spectrum white or dim red light treatment from 1100 to 1300 h daily during the first week after night-shift . Plasma melatonin ( for 24 h at the end of weeks 1 , 2 and 4 ) , and urinary 6-sulfatoxymelatonin ( aMT6s , for 48 h weekly ) were measured . A significant ( MANOVA ; p < 0.05 ) improvement in sleep was seen during night shift ( latency and duration ) and with bright light treatment ( latency ) . Melatonin and aMT6s rhythms delayed by 7 - 8 h during night-shift . The white light group readapted slowly , apparently by phase delay , as assessed by aMT6s measurement . The red light group readapted slightly , but significantly ( ANOVA , p < 0.01 ) faster than the white light group AIMS To evaluate the effects of a 24 h ( h )  physicians on-call duty ( OCD ) ( ' night shift ' ) on 24 h  electrocardiogram ( ECG ) , heart rate variability , blood pressure ( BP ) , and various biochemical serum and urine ' stress markers ' compared with a ' regular ' day at work . METHODS AND RESULTS The study was design ed as a prospect i ve r and omized cross-over trial with each physician completing a 24 h ( h ) OCD and a 24 h control period including a regular 8 h non-OCD .  Thirty healthy physicians with a median age of 33.5 years ( range 29.0 - 45.0 ) were analysed . Twenty-four hours  ECG and BP monitoring  were performed and participants were instructed to fill out an event diary and perform a 24 h urine collection . Furthermore , blood was drawn before and after OCD and control day . Twenty-four hours ECG showed a higher rate of ventricular premature beats ( VPB ) during early morning hours ( VPB 0 - 6 h , 0.5 vs. 0.0 , P = 0.047 ) and increased low-frequency normalized units ( 29.3 vs. 25.5 , P = 0.050 ) during night shift when compared with respective control night at home . During OCD , BP monitoring revealed a greater diastolic BP throughout 24 h ( 83.5 vs. 80.2 mmHg , P = 0.025 ) as well as during night-time ( 75.4 vs. 73.0 , P = 0.028 ) associated with a higher rate of systolic BP more than 125 mmHg during sleep time . Tumour necrosis factor alpha concentrations increased significantly during night shift ( 0.76 vs. 0.05 pg/mL , P = 0.045 ) . Urinary noradrenaline excretion was greater during OCD when compared with control day ( 46.0 vs. 36.0 microg/24 h , P = 0.007 ) . CONCLUSION Our results highlight the association of OCD with an increased risk profile for cardiovascular disease . In addition to the acute effects observed , frequent night-calls over a longer period possibly elicit sustained alterations in cardiovascular homeostasis Recent studies indicate a circadian rhythm in blood pressure and heart rate and its association with various neurotransmitters . In the present study , we examine the circadian nature of blood pressure/heart rate and salivary cortisol in night shift workers and whether these circadian changes produced by night shifts are reversible . Sixteen healthy nurses of both genders , aged 20 - 40 years , performing day and night shift duties , were r and omly selected out of 22 who volunteered for this study .  Ambulatory blood pressure monitoring  was done in all the subjects and  salivary cortisol levels were analyzed during both day and night shift duties . There were clinical ly significant changes in the Acrophase of blood pressure and cortisol levels , indicating ecphasia ( odd timing of systol 
    Output:
    
Nevertheless , we conclude that diurnal rhythms in cortisol , melatonin and HRV are not adapted to night work after 1–3 consecutive night shifts .