Nighttime Increase Research Articles

Air‐sea CO2 exchange in the equatorial Pacific

GasEx‐2001, a 15‐day air‐sea carbon dioxide (CO2) exchange study conducted in the equatorial Pacific, used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air‐sea CO2 fluxes. Direct covariance and profile method air‐sea CO2 fluxes were measured together with the surface ocean and marine boundary layer processes. The study took place in February 2001 near 125°W, 3°S in a region of high CO2. The diurnal variation in the air‐sea CO2 difference was 2.5%, driven predominantly by temperature effects on surface solubility. The wind speed was 6.0 ± 1.3 m s−1, and the atmospheric boundary layer was unstable with conditions over the range −1 < z/L < 0. Diurnal heat fluxes generated daytime surface ocean stratification and subsequent large nighttime buoyancy fluxes. The average CO2 flux from the ocean to the atmosphere was determined to be 3.9 mol m−2 yr−1, with nighttime CO2 fluxes increasing by 40% over daytime values because of a strong nighttime increase in (vertical) convective velocities. The 15 days of air‐sea flux measurements taken during GasEx‐2001 demonstrate some of the systematic environmental trends of the eastern equatorial Pacific Ocean. The fact that other physical processes, in addition to wind, were observed to control the rate of CO2 transfer from the ocean to the atmosphere indicates that these processes need to be taken into account in local and global biogeochemical models. These local processes can vary on regional and global scales. The GasEx‐2001 results show a weak wind dependence but a strong variability in processes governed by the diurnal heating cycle. This implies that any changes in the incident radiation, including atmospheric cloud dynamics, phytoplankton biomass, and surface ocean stratification may have significant feedbacks on the amount and variability of air‐sea gas exchange. This is in sharp contrast with previous field studies of air‐sea gas exchange, which showed that wind was the dominating forcing function. The results suggest that gas transfer parameterizations that rely solely on wind will be insufficient for regions with low to intermediate winds and strong insolation.

Comparison of ionospheric ionization measurements over Athens using ground ionosonde and GPS-derived TEC values

[1] GPS-derived TEC measurements (GPSTEC) represent the electron content up to 20,000 km, while a new ionosonde technique estimates the ionospheric electron content (ITEC) from ground ionosondes from the bottom of the ionosphere through the peak up to 1000 km. The two independent techniques were applied to determine ITEC and GPSTEC for Athens (38°N, 23.5°E) using the vertical Digisonde observations at Athens and GPSTEC maps produced by DLR/IKN for Europe using GPS measurements. Comparison of the two quantities over a 12-month time period shows a general agreement at daytime and a systematic deviation of ITEC towards lower values during nighttime. The diurnal variation of their residual values (ΔTEC) exhibits a morning minimum and an evening maximum, which can be explained in terms of ionospheric-plasmaspheric ionization exchange. The prominent evening enhancement, observed in all seasons around 1800–2000 LT, is attributed to the plasmaspheric bulge. The application of the superposed epoch analysis method on the daily ΔTEC values for several geomagnetic storms showed a rapid decrease of ΔTEC just after the storm initiation and a consecutive increase over a time period of 9 days, a behavior that is consistent with the plasmaspheric depletion and successive replenishment following storm activity. The daily variation of the ionospheric slab thickness is compatible with the variation of the thermospheric temperature within a day. Concerning the total slab thickness, this behavior is altered by the nighttime increase, which is most prominent in winter, and it is due to the lowering of the O+/H+ transition height. In summary, this analysis presents additional proof that the residuals ΔTEC = GPSTEC − ITEC provide information about the qualitative characteristics of the plasmaspheric dynamics as deduced from their diurnal behavior and their variation during geomagnetic storms.

Transitory increased blood pressure after upper airway surgery for snoring and sleep apnea correlates with the apnea-hypopnea respiratory disturbance index.

A transitory increase in blood pressure (BP) is observed following upper airway surgery for obstructive sleep apnea syndrome but the mechanisms implicated are not yet well understood. The objective of the present study was to evaluate changes in BP and heart rate (HR) and putative factors after uvulopalatopharyngoplasty and septoplasty in normotensive snorers. Patients (N = 10) were instrumented for 24-h ambulatory BP monitoring, nocturnal respiratory monitoring and urinary catecholamine level evaluation one day before surgery and on the day of surgery. The influence of postsurgery pain was prevented by analgesic therapy as confirmed using a visual analog scale of pain. Compared with preoperative values, there was a significant (P < 0.05) increase in nighttime but not daytime systolic BP (119 5 vs 107 3 mmHg), diastolic BP (72 4 vs 67 2 mmHg), HR (67 4 vs 57 2 bpm), respiratory disturbance index (RDI) characterized by apnea-hypopnea (30 10 vs 13 4 events/h of sleep) and norepinephrine levels (22.0 4.7 vs 11.0 1.3 g l-1 12 h-1) after surgery. A positive correlation was found between individual variations of BP and individual variations of RDI (r = 0.81, P < 0.01) but not between BP or RDI and catecholamines. The visual analog scale of pain showed similar stress levels on the day before and after surgery (6.0 0.8 vs 5.0 0.9 cm, respectively). These data strongly suggest that the cardiovascular changes observed in patients who underwent uvulopalatopharyngoplasty and septoplasty were due to the increased postoperative RDI.

Analytical representation of the plasmasphere electron temperature distribution based on Akebono data

A new approach is used to reveal the average thermal structure of plasmasphere at altitudes between 1000 and 10,000 km. This paper only considers the plasmasphere inside L = 3, and this region of space is sliced at L = 2 to one internal (denoted as equatorial) and one external (denoted as midlatitude) part. Each of these parts is divided into four altitude/local time zones: northern and southern daytime and northern and southern nighttime. Electron temperature (Te) distribution measured along every individual orbit within each zone is approximated by simple expressions that are functions of the geomagnetic latitude or L. The fitting is performed independently to each zone, and two coefficients are extracted from the individual fits: Te value at the equatorward border and the coefficient scaling the latitudinal shape. These coefficients are accumulated in each zone in order to obtain its average Te structure as a function of altitude. The fitting coefficients exhibit large scatter, which reflects the large day‐to‐day variability of plasmasphere Te. The fitting error is estimated at 6% and 12% for the equatorial and midlatitude zones, respectively. The average fit coefficients are used to check for seasonal variations in each zone and for a possible hemispheric asymmetry. It is determined that the winter sunlit plasmasphere is hotter than the summer one; the case for nighttime is reverse. The day/night amplitude is larger in the winter than in the summer, and the seasonal differences are more pronounced in the Southern Hemisphere. The latitude gradients of Te reveal a hemispheric asymmetry, which is largest during northern daytime winter. Solar activity is found to affect the plasmasphere Te. During the day at 3000 km and L = 2, Te increases by ∼2000°K when F10.7 changes from 70 to 300. The nighttime increase is ∼1000°K. At the equator, Te shows weaker dependence on solar activity. The average heat flux through 3000 km altitude is estimated at 3.7 × 109 (eV cm−2 s−1) during the day and 1.6 × 108 (eV cm−2 s−1) during the night.

Daily rhythm of glutathione peroxidase activity, lipid peroxidation and glutathione levels in tissues of pinealectomized rats

Melatonin is a component of the antioxidant defense system since it has radical scavenging and antioxidant activities. In the present study, we aimed to investigate the endogenous rhythm of antioxidant enzyme glutathione peroxidase (GSH-Px) activity, oxidized glutathione (GSSG) and lipid peroxidation levels in tissues of pinealectomized rats (PINX). Rats were sacrificed by decapitation at 4 h intervals. GSH-Px activity, GSSG and lipid peroxidation levels showed a daily rhythm both in controls and in PINX rats. GSH-Px and GSSG exhibited the peak levels after the peak time of melatonin which was determined previously by other groups. Lipid peroxidation levels increased progressively during the night and started to decline before the GSH-Px peak time. These findings suggest that endogenous melatonin is involved in the night time increase of GSH-Px activity and GSSG levels and modulates the daily rhythm pattern of GSH-Px. In conclusion, pinealectomy which eliminates the melatonin rhythm has a supressor effect on GSH-Px activity levels.

Endogenous rhythms of melatonin, total antioxidant status and superoxide dismutase activity in several tissues of chick and their inhibition by light.

Melatonin was recently shown to be a component of the antioxidative defense system of organisms due to its free radical scavenging ability and to its capacity to stimulate several antioxidant enzymes. In this report, we studied the endogenous rhythm of the antioxidant enzyme superoxide dismutase (SOD) in three different tissues (cerebral cortex, liver and lung) of chick (Gallus domesticus) (three weeks, at age and sacrificed every 2 hr). During the study the chicks were under a light:dark cycle of 12:12. Total antioxidant status of the plasma was correlated with physiological blood melatonin concentrations. Superoxide dismutase activity exhibited a marked 24 hr rhythm in cerebral cortex, lung and liver, with peak activity coincident with the melatonin and total antioxidant status peaks. The exposure of chicks to constant light for 7 days eliminated the melatonin rhythm as well as the peaks in superoxide dismutase activity and the total antioxidant status. These findings suggest that the melatonin rhythm may be related to the nighttime increase in the superoxide dismutase activity and to total antioxidant capacity of the blood.

Nighttime increases in ionosphere electron content (a statistical and experimental study)

Variations of nighttime ionosphere total electron content (TEC) have been studied using the Faraday rotation angle data obtained at middle latitudes (Almaty, Kazakhstan) from the geostationary satellite ETS-II during the years 1985–1986. Critical frequency values of the ionosphere F2-layer shown in the vertical sounding ionograms measured at Alma-Ata station (43.25°N, 76.92°E) have been used throughout the study. Morphological and statistical analyses are used to investigate the anomalous nighttime increases (ANI) in TEC that occurred during the years. The ANI events are looked at in terms of their amplitude and probability of occurrence, and their duration and peak time. Special features in the events are found to interpret the ANI as a travelling ionosphere disturbance. By examining the hourly, nightly values foF2 and TEC over the two years it has been suggested that the ANI events exist on an ionosphere background which contains regular quasi-periodical oscillations in the period range of 4–30 days.

Is the Resting Rate of Oxygen Consumption of Locomotor Muscles in Crustaceans Limited by the Low Blood Oxygenation Strategy?

Numerous water-breathers exhibit a gas-exchange regulation strategy that maintains O(2) partial pressure, P(O2), in the arterial blood within the range 1-3 kPa at rest during the daytime. In a night-active crustacean, we examined whether this could limit the rate of O(2 )consumption (M(O2)) of locomotor muscles and/or the whole body as part of a coordinated response to energy conservation. In the crayfish Astacus leptodactylus, we compared the in vitro relationship between the M(O2) of locomotor muscles as a function of the extracellular P(O2) and P(CO2) and in vivo circadian changes in blood gas tensions at various values of water P(O2). In vitro, the M(O2) of locomotor muscle, either at rest or when stimulated with CCCP, was O(2)-dependent up to an extracellular P(O2) of 8-10 kPa. In vivo, the existence of a night-time increase in arterial P(O2) of up to 4 kPa at water P(O2) values of 20 and 40 kPa was demonstrated, but an experimental increase in arterial P(O2) during the day did not lead to any rise in whole-body M(O2). This suggested that the low blood P(O2) in normoxia has no global limiting effect on daytime whole-body M(O2). The participation of blood O(2) status in shaping the circadian behaviour of crayfish is discussed.

On the mechanism of the post-midnight winter &amp;lt;i&amp;gt;N&amp;lt;sub&amp;gt;m&amp;lt;/sub&amp;gt;F&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt; enhancements: dependence on solar activity

Abstract. The mechanism of the NmF2 peak formation at different levels of solar activity is analyzed using Millstone Hill IS radar observations. The hmF2 nighttime increase due to thermospheric winds and the downward plasmaspheric fluxes are the key processes responsible for the NmF2 peak formation. The electron temperature follows with the opposite sign the electron density variations in this process. This mechanism provides a consistency with the Millstone Hill observations on the set of main parameters. The observed decrease of the nighttime NmF2 peak amplitude with solar activity is due to faster increasing of the recombination efficiency compared to the plasmaspheric flux increase. The E×B plasma drifts are shown to be inefficient for the NmF2 nighttime peak formation at high solar activity.Key words: Ionosphere (ionosphere-atmosphere interactions; mid-latitude ionosphere; plasma temperature and density)

Seasonal and daily variations in plasma melatonin in the high-arctic Svalbard ptarmigan (Lagopus mutus hyperboreus).

This study presents the daily rhythm of melatonin secretion throughout one year in a bird from the northern hemisphere, the Svalbard ptarmigan (Lagopus mutus hyperboreus), which lives naturally at 76-80 degrees N. Eight Svalbard ptarmigan were caged outdoors at 70 degrees N and blood sampled throughout one day each month for 13 months. At this latitude, daylight is continuous between May and August, but there is a short period of civil twilight around noon from late November to mid January. There was no daily rhythm in plasma melatonin in May-July. Plasma melatonin levels varied significantly throughout the day in all other months of the year, with the nighttime increase reflecting the duration of darkness. The highest mean plasma concentration occurred at midnight in March (110.1 +/- 16.5 pg/ml) and represented the annual peak in estimated daily production. Around the winter solstice, melatonin levels were significantly reduced at noon and elevated during the nearly 18 h of consecutive darkness, and the estimated mean daily production of melatonin was significantly reduced. Thus, at the times of the year characterized by light-dark cycles, melatonin may convey information concerning the length of the day and, therefore, progression of season. The nearly undetectable low melatonin secretion in summer and the reduced amplitude and production in midwinter indicate a flexible circadian system that may reflect an important adaptation to life in the Arctic.

Photoneural regulation of rat pineal hydroxyindole-O-methyltransferase (HIOMT) messenger ribonucleic acid expression: an analysis of its complex relationship with HIOMT activity.

In the pineal gland, synthesis of melatonin requires O-methylation catalyzed by hydroxyindole-O-methyltransferase (HIOMT; EC 2.1.1.4). We investigated in vivo the molecular mechanisms involved in the regulation of rat pineal HIOMT messenger RNA (mRNA) expression and activity using in situ hybridization and radioenzymatic assay. HIOMT mRNA levels and activity are both detectable during the daytime and display nocturnal increases of 100% and 30%, respectively. These variations are controlled by the endogenous clock, as they persist in constant darkness. The nocturnal increase in HIOMT mRNA mainly results from a beta-adrenergic stimulation of HIOMT gene expression without requiring de novo synthesis of a transcription factor. In contrast, the nocturnal increase in HIOMT activity appears independent of beta1/alpha1-adrenergic stimulation. A light pulse at night abolishes the nighttime increase in HIOMT mRNA, but not HIOMT activity. Constant light application for up to 11 days does not depress HIOMT mRNA levels lower than the daytime levels, but decreases enzyme activity down to 50% of the daytime level. This finding indicates that the nocturnal stimulation of HIOMT gene expression is required for sustaining a basal level of activity over a few days. Our data suggest 1) that HIOMT gene expression is partly regulated by beta1-stimulation; and 2) that HIOMT activity is regulated over the short term by a nonnoradrenergic stimulus and over the long term by noradrenergic stimulation.

Some F2-layer effects during the January06-11,1997 cedar storm period as observed with theMillstoneHill incoherent scatter facility

An observed daytime negative storm effect on 10 January and strongpost-midnight N m F 2 increases on 7 and 8 January followed by a steep T e decrease down to T n are analyzed. Daytime neutral composition and T n(h) were inferred from incoherent scatter (IS) radar observations using aself-consistent approach to F 2 -layer modelling. Observed daytime T e , T i , N e profiles were corrected for the disturbed day using the calculated ioncomposition. The main reason for the observed N m F 2 negative storm effectis the O/(N 2 O 2 ) ratio decrease in the thermosphere. The calculated O/N 2 ratio at hm F 2 is less by a factor of 3.7 than MSIS prediction for 10January and is close to MSIS for quiet days. Strong Nm F 2 night-time increase by afactor of 3 is due to a plasma compression mechanism under the action of observed EB drift(westward E-field) moving plasma from higher L shells to lower ones and squeezing it into the F 2 -region. Moderate (1-2)10 8 cm -2 s -1 O fluxes in agreement with IS observations were shown to be sufficient to account for the night-timeN m F 2 increase. This elevation in the F 2 -region electron densityis shown to be the main reason for the observed steep Te post-midnight decrease. 1999 ElsevierScience Ltd. All rights reserved.

Development of Temperature Patterns during Clear Nights

Abstract This paper examines nocturnal air temperature structure development in complex terrain. Clear nights were studied in order to compare the cooling rate in different topographical areas with a variety of land cover. It was found that large variations in temperature developed over a very short time period after sunset and that in situ cooling was the dominant factor causing this in valleys and other wind-sheltered locations. Shelter can be provided both from valley sides and from nearby trees with the main effect being to reduce the vertical mixing. The nighttime increase in temperature difference between valley bottoms and nearby reference areas was interpreted to be due to cold airflows. This was also shown by the increasing lateral extension of cold air accumulating in valleys. This development was found only in open valley locations. Sheltered areas cooled at a much faster rate than exposed sites during early evening. Further cooling did not increase the lateral extension of cold air or result i...

Melatonin: role in gating nocturnal rise in sleep propensity.

The present article reviews the evidence that melatonin possesses sleep-inducing effects and that it gates the increase in nocturnal sleepiness. It is shown that, without exception, all the studies that have investigated daytime administrations of melatonin reported increased sleepiness, even at doses that do not increase plasma levels of melatonin beyond its physiological levels. By contrast, nighttime increase in sleepiness was achieved only after administration of high doses. Based on these findings and on the precise coupling between the endogenous nocturnal increase in melatonin secretion and the opening of the sleep gate, it is suggested that melatonin participates in the regulation of the sleep-wake cycle by inhibiting the central nervous system wakefulness generating system. This inhibition allows a smooth transition from wakefulness to sleep. Clinical findings on decreased levels of nocturnal melatonin in chronic insomniacs, and on the efficacy of exogenous melatonin in improving sleep in melatonin-deficient insomniacs, are congruent with this hypothesis.

Electron density profile changes in a pre-earthquake period

Recent investigations show that there are a number of specific effects at F-region altitudes which could be considered as earthquakes ionospheric precursors. One of such effects is a night-time increase of F2 critical frequency 1 to 2 days before the seismic event. Distortions of the electron density profiles which occur at the same time are investigated in this report. N(h)-profiles obtained from vertical soundings near the epicenter differ noticeably from IRI predictions.

A DOAS study on the origin of nitrous acid at urban and non-urban sites

Measurements have been performed to study the origin of the tropospheric nighttime concentrations of HNO 2 The experiments were carried out at three locations with different levels of pollution by using a commercial differential optical absorption spectroscopy (DOAS) and a point sampling analyser. The DOAS observations allowed study of the concentrations of HNO 2, NO 2, SO 2 and O 3. Light intensities given by the DOAS system were used to estimate rough values for the total surface of aerosols S along the beam path. NO was measured by a chemiluminescent monitor. A HNO 2 formation is clearly indicated by the nighttime increase of the HNO 2/NO x ratio. Correlation studies suggest the participation of the aerosol surface in the heterogeneous formation in the less polluted areas. In Milan the measured HNO 2 concentrations are considered to be influenced by the nighttime boundary layer and the heterogeneous formation on permanent surfaces like buildings.

Modeling studies of equatorial plasma fountain and equatorial anomaly

The importance of diffusion, electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) for the ionosphere above Jicamarca (77°W) under magnetically quiet ( Ap = 4) equinoctial conditions (day 264) at medium solar activity ( F10.7 = 145). The study also investigates the effects of the fountain, which include the equatorial anomaly. The F-region vertical E × B drift velocity measured at the equatorial station Jicamarca is used to represent the electrodynamic drift. The neutral wind is obtained from the HWM90 thermospheric wind model. As expected, the F-region electrodynamic drift generates the plasma fountain and the anomaly, which are symmetric with respect to the equator. The neutral wind makes the fountain and the anomaly asymmetric, with larger plasma flow (towards the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. The paper also addresses many important (some new) features which are related to the fountain. The features are: (1) the possibility of existence of an additional layer (called the G-layer) in the equatorial ionosphere, (2) the reverse plasma fountain, (3) the equatorial anomaly in vertical ionospheric electron content (IEC), (4) the presence (in Nmax) and absence (in IEC) of noon bite-out, (5) the occurrence of nighttime increase in ionization, and (6) plasma bubbles and spread- F.

Power spectral analysis of heart period variability in hypertensive patients with left ventricular hypertrophy

This study aimed to characterize sympathovagal balance by heart period power spectrum analysis in hypertensive patients with echocardiographic evidence of left ventricular hypertrophy. Twenty ambulatory patients (11 men and 9 women), aged 50 +/- 10 years, with established essential hypertension and echocardiographic left ventricular hypertrophy, performed 24-h blood pressure monitoring and electrocardiogram Holter recording on 2 consecutive days. Twenty age- and sex-matched normal subjects comprised the control group. Power spectrum analysis, performed using the fast Fourier transform algorithm, demonstrated lower values of low and high frequency power in hypertensives than in controls, while ultralow and very low frequency power were similar in the two groups. Very low frequency, low frequency, and high frequency power increased during the night in both groups, showing a similar circadian pattern. We found a direct correlation between daytime systolic (r = 0.51; P < .05) and diastolic (r = 0.52; P < .05) blood pressure and left ventricular mass index. Moreover, negative correlations were found between left ventricular mass index and low frequency (r = -0.47; P < .05) and high frequency power (r = -0.47; P < .05). There was a direct correlation between nighttime decrease in systolic blood pressure and nighttime increase in high frequency power (r = 0.45; P < .05). As 24-h low frequency and high frequency power, obtained using the Fourier transform algorithm, both reflect the parasympathetic modulation of heart rate, our results demonstrate that hypertensive patients with left ventricular hypertrophy are characterized by a sympathovagal imbalance with a reduction of vagal tone that is more evident with increasing severity of hypertension.

Equatorial plasma fountain and its effects: Possibility of an additional layer

The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere‐ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E×B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about ±30° magnetic latitude; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within ±10° of the magnetic equator during the prenoon hours when the drift is large. At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests.

Effects of aging on 24-h dynamic baroreceptor control of heart rate in ambulant subjects

The effects of aging on the dynamic modulation of baroreflex sensitivity over 24 h was assessed in eight elderly (mean age +/- SD, 63.9 +/- 3.2 yr) and in eight young (23.9 +/- 6.1 yr) mild or moderate essential hypertensive patients, who were subject to a 24-h intra-arterial (Oxford technique) blood pressure recording in ambulatory conditions. The sensitivity of baroreflex control of the heart rate was dynamically assessed by quantifying 1) the slope of the regression line between pulse interval (the reciprocal of heart rate) and systolic blood pressure changes over spontaneously occurring hypertension-bradycardia or hypotension-tachycardia sequences (time domain analysis) and 2) the ratio between spectral-powers of pulse interval and systolic blood pressure around 0.1 Hz (alpha-coefficient: frequency domain analysis). The 24-h average sequence slope was lower in old than in young individuals (4.4 +/- 0.5 vs. 9.9 +/- 1.3 and 4.8 +/- 0.7 vs. 8.4 +/- 1.4 ms/mmHg for hypertension-bradycardia and hypotension-tachycardia sequences, respectively; P < 0.05 for both). Similar results were obtained by using the alpha-coefficient approach. The marked nighttime increase in baroreflex sensitivity observed in young individuals was much less evident in the elderly. Thus 24-h baroreflex sensitivity is markedly impaired by aging. The impairment becomes manifest also as an inability to increase baroreflex sensitivity at night.