***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 OBJECTIVE To assess as whether insomniacs have higher nighttime blood pressure ( BP ) and a blunted day-to-night BP reduction , recognized markers of increased risk of cardiovascular morbidity and mortality . DESIGN Prospect i ve case-control study . SETTING University hospital-based sleep research laboratory . PARTICIPANTS Thirteen normotensive subjects with chronic primary insomnia ( 9 women , 42 + /- 7 y ) and 13 sex- and age-matched good sleepers . MEASUREMENTS AND RESULTS Subjects underwent 2-week sleep diary and 3 sleep studies to provide subjective and objective sleep variables , and 24-h beat-to-beat BP recording to provide daytime , night-time and day-to-night BP changes ([nighttime-daytime]/daytime)*100 ) ( BP dipping ) . Spectral analysis of the electroencephalogram ( EEG )  was also performed during sleep of night 3 to assess EEG activity in the beta frequency ( 16 - 32 Hz ) , a measure of brain cortical activation . Nighttime SBP was higher ( 111 + /- 15 vs 102 + /- 12 mm Hg , P < 0.01 ) and day-to-night SBP dipping was lower (-8 % + /- 6 % vs -15 % + /- 5 % , P < 0.01 ) in insomniacs than good sleepers . Insomniacs also had higher activity in EEG beta frequency ( P < 0.05 ) . Higher nighttime SBP and smaller SBP dipping were independently associated with increased EEG beta activity ( P < 0.05 ) . CONCLUSIONS Higher nighttime SBP and blunted day-to-night SBP dipping are present in normotensive subjects with chronic insomnia and are associated with a hyperactivity of the central nervous system during sleep . An altered BP profile in insomniacs could be one mechanism implicated in the link between insomnia and cardiovascular morbidity and mortality documented in epidemiological studies Entropy , as it relates to dynamical systems , is the rate of information production . Methods for estimation of the entropy of a system represented by a time series are not , however , well suited to analysis of the short and noisy data sets encountered in cardiovascular and other biological studies . Pincus introduced  approximate entropy ( ApEn ) , a set of measures of system complexity closely related to entropy , which is easily applied to clinical cardiovascular and other time series . ApEn statistics , however , lead to inconsistent results . We have developed a new and related complexity measure ,  sample entropy ( SampEn )  , and have compared ApEn and SampEn by using them to analyze sets of r and om numbers with known probabilistic character . We have also evaluated cross-ApEn and cross-SampEn , which use cardiovascular data sets to measure the similarity of two distinct time series . SampEn agreed with theory much more closely than ApEn over a broad range of conditions . The improved accuracy of SampEn statistics should make them useful in the study of experimental clinical cardiovascular and other biological time series Objectives : The prevalence of insomnia is very high in our society . Although pharmacological treatment of insomnia is available , most hypnotics have been shown to alter sleep architecture and have many adverse effects . Gabapentin was originally design ed for antiepileptic therapy ; however , some studies reported that its use increases slow-wave sleep in healthy volunteers or patients . Our goal was to evaluate the benefits of gabapentin in the treatment of primary insomnia in patients . Methods : Eighteen patients with primary insomnia participated in the study . They received gabapentin treatment for at least 4 weeks . All patients received  polysomnography , a biochemical blood test , and neuropsychological tests before and after the treatment period . All measures were analyzed with Student t test to examine the treatment effects of gabapentin , except that the measures of heart rate variability were analyzed with analysis of variance . Results : Polysomnographic study revealed increased sleep efficiency and slow-wave sleep , decreased wake after sleep onset , and spontaneous arousal index after gabapentin treatment . The biochemical blood test revealed decreased prolactin levels in the morning after treatment . Electroencephalographic power spectral analysis showed increased delta-2 and theta power in sleep stage 1 and decreased sigma activity power in sleep stages N2 and N3 after gabapentin treatment . Heart rate variability analyses also showed a significant increase in normalized high frequency percentage in sleep stages N2 and N3 and low frequency-high frequency ratio in sleep stage N2 after treatment . In addition , neuropsychological tests revealed the elevation of visual motor processing speed after gabapentin treatment . Conclusions : Gabapentin enhances slow-wave sleep in patients with primary insomnia . It also improves sleep quality by elevating sleep efficiency and decreasing spontaneous arousal . The results suggest that gabapentin may be beneficial in the treatment of primary insomnia STUDY OBJECTIVES To examine the joint effect of insomnia and objective short sleep duration on hypertension risk . DESIGN Representative cross-sectional study . SETTING Sleep laboratory . PARTICIPANTS 1,741 men and women r and omly selected from central Pennsylvania . INTERVENTIONS None . MEASUREMENTS Insomnia was defined by a complaint of insomnia with a duration > or = 1 year , while poor sleep was defined as a complaint of difficulty falling asleep , staying asleep , or early final awakening .  Polysomnographic sleep duration  was classified into 3 categories : > or = 6 h sleep ( top 50 % of the sample ) ; 5 - 6 h ( approximately the third quartile of the sample ) ; and < or = 5 h ( approximately the bottom quartile of the sample ) . Hypertension was defined based either on blood pressure measures or treatment . We controlled for age , race , sex , body mass index , diabetes , smoking , alcohol use , depression , sleep disordered breathing ( SDB ) , and sampling weight . RESULTS Compared to the normal sleeping and > 6 h sleep duration group , the highest risk of hypertension was in insomnia with < 5 h sleep duration group ( OR [ 95 % CI ] 5.1 [ 2.2 , 11.8 ] ) , and the second highest in insomnia who slept 5 - 6 hours ( OR 3.5 [ 1.6 , 7.9 ] P < 0.01 ) . The risk for hypertension was significantly higher , but of lesser magnitude , in poor sleepers with short sleep duration . CONCLUSIONS Insomnia with short sleep duration is associated with increased risk of hypertension , to a degree comparable to that of other common sleep disorders , e.g. , SDB . Objective sleep duration may predict the severity of chronic insomnia a prevalent condition whose medical impact has been apparently underestimated Background Clinical presentation phenotypes of obstructive sleep apnoea ( OSA ) and their association with comorbidity as well as impact on adherence to continuous positive airway pressure ( CPAP ) treatment have not been established . Methods A prospect i ve follow-up cohort of adult patients with OSA ( apnoea-hypopnoea index ( AHI ) of ≥5/h ) from 17 European countries and Israel ( n = 6,555 ) was divided into four clinical presentation phenotypes based on daytime symptoms labelled as  excessive daytime sleepiness ( “ EDS ” ) and nocturnal sleep problems other than OSA ( labelled as “ insomnia ” ) : 1 ) EDS ( daytime+/nighttime- ) , 2 ) EDS/insomnia ( daytime+/nighttime+ ) , 3 ) non-EDS/non-insomnia ( daytime-/nighttime- ) , 4 ) and insomnia ( daytime-/nighttime+ ) phenotype . Results The EDS phenotype comprised 20.7 % , the non-EDS/non-insomnia type 25.8 % , the EDS/insomnia type 23.7 % , and the insomnia phenotype 29.8 % of the entire cohort . Thus , clinical presentation phenotypes with insomnia symptoms were dominant with 53.5 % , but only 5.6 % had physician diagnosed insomnia . Cardiovascular comorbidity was less prevalent in the EDS and most common in the insomnia phenotype ( 48.9 % vs. 56.8 % , p<0.001 ) despite more severe OSA in the EDS group ( AHI 35.0±25.5/h vs. 27.9±22.5/h , p<0.001 , respectively ) . Psychiatric comorbidity was associated with insomnia like OSA phenotypes independent of age , gender and body mass index ( HR 1.5 ( 1.188–1.905 ) , p<0.001 ) . The EDS phenotype tended to associate with higher CPAP usage ( 22.7 min/d , p = 0.069 ) when controlled for age , gender , BMI and sleep apnoea severity . Conclusions Phenotypes with insomnia symptoms comprised more than half of OSA patients and were more frequently linked with comorbidity than those with EDS , despite less severe OSA . CPAP usage was slightly higher in phenotypes with EDS INTRODUCTION Impaired heart rate variability ( HRV ) is associated with poor outcome in diabetic patients . The present prospect i ve study compared spectral components of HRV obtained by either fast Fourier transform ( FFT ) or autoregressive ( AR ) analyses in diabetic patients . METHODS Thirty patients ( 49+/-12 years ; 11 F/19 M ; 60 % with insulin-dependent type 1 diabetes ) underwent 24-h ambulatory electrocardiographic recordings which comprised a 10-min resting period at the onset ( n=30 ) and end ( n=12 ) of the monitoring . Spectral analysis was applied to 5-min sequences at rest , and the total power and power spectra integrated over the very low ( VLF ) , low ( LF ) , and high ( HF ) frequency b and s were obtained . RESULTS Fifteen patients had moderately depressed HRV and two patients had highly depressed HRV ( st and ard deviation of the RR intervals over 24-h<100 ms and < 50 ms , respectively ) . Both raw data and ln-transformed data were significantly different between FFT and AR . All spectra component were obtained in each patient using FFT . Using AR , the LF/HF ratio could not be estimated or was zero in 4 and 11 patients , respectively . The AR results were more sensitive than FFT results to minor changes ( + /-5 % ) in the timing of the onset of analysis . The day-to-day reproducibility of FFT was better than that of AR . Finally , using FFT , the LF/HF ratio , LFnu , and HFnu were essentially redundant ( nu = normalized units ) . CONCLUSIONS The spectral components of short-term HRV calculated by using the FFT and AR methods were not interchangeable and FFT analysis must be preferred in diabetic patients AIMS Heart rate variability has been proposed as an indicator of cardiovascular health . Since women have a lower cardiovascular risk , we hypothesized that there are gender differences in autonomic modulation . METHODS AND RESULTS In 276 healthy subjects ( 135 women , 141 men ) between 18 and 71 years of age , 24 h heart rate and heart rate variability were determined . All heart rate variability parameters , except for pNN50 and high frequency power , were higher in men . After adjustment for heart rate , we obtained gender differences for : the st and ard deviation ( P=0.049 ) , the st and ard deviation of the 5 min average ( P=0.047 ) , low frequency power ( absolute values , P=0.002 ; normalized units , P<0.001 ) and ratio low frequency/high frequency ( P<0.001 ) . There were no significant gender differences in heart rate variability parameters denoting vagal modulation . Gender differences were confined to age categories of less than 40 years of age . The majority of heart rate variability parameters decreased with age . Only in men , was a higher body mass index associated with a higher heart rate and with lower heart rate variability parameters ( P<0.001 ) . CONCLUS 
    Output:
    
Autonomic regulation tends to be consistent between insomniacs , as long as they are grouped according to their respective phenotype , as shown in the insomnia subgroup with objective ly short sleep duration .
Our hypothesis is that these differences in the expression of cardiovascular activity could be explained by the heterogeneity of the disorder .
Therefore , the determination of insomnia phenotypes , and the study of cardiovascular measures , rather than heart rate variability alone , will give more insight into the link between insomnia and cardiovascular regulation .
This study suggests that cardiovascular activity differs between insomniacs and controls .