How human sleep in space--investigations during space flights.

Sleep problems have been observed during many of the space flights. The existence of poor quality of sleep, fatigue, insomnia or different alterations in sleep structure, organization and sleep cyclicity have been established. Nevertheless results obtained from investigations of human sleep on board manned space vehicles show that it is possible to keep sleep patterns related to the restorative and adaptive processes. For the first time in the frame of the "Intercosmos" program a multi-channel system for recording and analysis of sleep in space was constructed by scientists of the Bulgarian Academy of Sciences and was installed on board the manned Mir orbiting station. In 1988 during the joint Bulgarian-Russian space flight continues recording of electro-physiological parameters necessary to estimate the sleep stages and sleep organization was made. These investigations were continued in next space flights of different prolongation. The results were compared with the findings obtained under the conditions during the pre- and post-flight periods.

Evaluation of sleep in space flight.

NASA: Researchers evaluated the sleep of astronauts onboard the Mir space station to determine the effect of weightlessness on sleep. Results indicate that all sleep stages occur in weightlessness, but vary in timing and duration. It is noted that individual physiological features of the astronauts play a marked role in sleep structure.^ieng

Analysis of heart rate variability during short-term space flight.

The heart rate (h.r.) of normal healthy subjects varies in rest, under high-load conditions and in pathology. The heart rate variability (h.r.v.) integrates many mechanisms in- and outside the central nervous system, which influence and regulate the h.r. Statistical methods have been used to obtain additional information concerning the diagnostic capabilities h.r.v., the correlation between ECG R-R intervals and their variability and the prognostic significance of h.r.v. The benefits of modification of h.r.v. are not well known. In our previous study the h.r.v. of astronauts before, during and after long-term space flight (seven months) was examined. In this study we make analysis of h.r.v. in wake and sleep in different phases during a short-time space flight (9 days) of the Bulgarian astronaut A.A. in 1988.

Estimation through experimental data of model parameters of the anterior pituitary LH response.

A mathematical model of the luteinizing hormone (LH) response of the anterior pituitary to the gonadotropin releasing hormone (GnRH) stimulation has been proposed by Yanev et al., 1989. The experimental data available are poor. Usually the output relation LH (GNRH) and only a few of the values of the model parameters are experimentally known while the values of the resting parameters are chosen in the course of computer simulations so that a satisfactory agreement between the experimental and the simulation LH outputs is obtained. Such an agreement is an indicator that the ratios between the simulation values are possibly properly chosen but it does not means categorically that their absolute simulation values are the proper ones. In this work it has been proved that once this agreement is attained and enough experimental values of the model parameters are known then the values of the resting model parameters can be calculated (scaled) making use of their simulation values. With the scaled values introduced in the model the agreement obtained between the model and the experimental LH output functions is reserved. The scaled values can be interpreted as estimations of the native values of the respective model parameters.

Optimal interelectrode distance with the method of source density analysis.

Three error sources which contribute to the total error of the second space derivative (SSD) used with the method of current source density analysis (CSD) were examined. The approximate formula usually used with CSD was the first error source. Another error originated from the finite interelectrode distance. The measurement error of the device with which the potential was recorded was the third error source herein considered. A relative SSD error was determined to estimate the effect of the errors examined. The experimental data (a section from the three-dimensional potential profile in annuran cerebellum produced by parallel fiber activation) were fitted by polynomials, Fourier expansion and cubic splines. Then a preferable interelectrode distance was obtained corresponding to the minimal relative error.

A probability concept about size distributions of sonicated lipid vesicles.

The sonication procedure of preparation of small unilamellar vesicles is modelled as a process of uniform random fragmentation of the lipid aggregates. The vesicle size distribution evolving in this process is shown to be identical with the Weibull extremal probability distribution. Size histograms of sonicated small vesicles of various phospholipid composition were obtained by using electron microscopy (negative staining). Their successful simulation with Weibull curves shows that theory agrees with experiment. A similarly good agreement is found also with size histograms obtained by freeze-fracture of phosphatidylcholine-cholesterol vesicles (Van Venetië, R., Leunissen-Bijvelt, J., Verkleij, A.J. and Ververgaert, P.H.J.T. (1980) J. Microsc. 118, 401-408). This analysis allows a refinement of some earlier conclusions about the effect of cholesterol on the size of the sonicated vesicles. It follows from the theoretical model that the only intrinsic characteristic of the sonicated vesicles is the lower limit of their size. The other characteristics of the size distribution such as expectancy, dispersion, position and height of the maximum depend on the intensity of fragmentation. It is concluded that the size distribution of sonicated small vesicles is completely determined by the procedure of their preparation and, therefore, the condition of thermodynamic equilibrium between aggregated and monomeric lipid is irrelevant in this case.