Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights

Sleep, circadian rhythm, and neurobehavioral performance measures were obtained in five astronauts before, during, and after 16-day or 10-day space missions. In space, scheduled rest-activity cycles were 20-35 min shorter than 24 h. Light-dark cycles were highly variable on the flight deck, and daytime illuminances in other compartments of the spacecraft were very low (5.0-79.4 lx). In space, the amplitude of the body temperature rhythm was reduced and the circadian rhythm of urinary cortisol appeared misaligned relative to the imposed non-24-h sleep-wake schedule. Neurobehavioral performance decrements were observed. Sleep duration, assessed by questionnaires and actigraphy, was only approximately 6.5 h/day. Subjective sleep quality diminished. Polysomnography revealed more wakefulness and less slow-wave sleep during the final third of sleep episodes. Administration of melatonin (0.3 mg) on alternate nights did not improve sleep. After return to earth, rapid eye movement (REM) sleep was markedly increased. Crewmembers on these flights experienced circadian rhythm disturbances, sleep loss, decrements in neurobehavioral performance, and postflight changes in REM sleep.     

Phase shift in leaf movements of xanthium attributed to age and rhythm patterns

Leaves of cockelbur (Xanthium strumarium L.) have been reported to be in either an upright or downward position during the dark span (night) of a 24-hour cycle. Results from our studies clearly indicate that such differences in leaf position are not related to differences in ecotypes but can be attributed to age of the leaf, pattern of the waveform of the rhythm at various stages of the light-dark synchronizer regimen, and the statistical model used for the analysis of the waveform. Younger leaves reached a maximum upright position closer to the middle of the dark span, whereas older leaves reached this position closer to the end of the dark span. A phase shift of up to 6 to 10 hours may occur as the leaf ages. Results from the examination of the pattern of the waveform at four different times showed that the pattern of a younger leaf was different from that of an older leaf during the middle of the dark span, during the light-to-dark transition, and during the middle of the light span, but not during the dark-to-light transition. Linear regression, statistical analyses, and the fitting of harmonics clearly indicate that it is the trough, more than the peak, that differs with the age of the leaf.     

The effects of freeze-thaw cycles and leaching on the loss of soluble carbohydrates from leaf material of two subantarctic plants

Summary Leaves and litter of two phanerogams (Acaena magellanica (Lam.) Vahl and Poa flabellata (Lam.) Hook. f.) were collected in spring on the subantarctic island of South Georgia. Leaves immersed in water lost up to 80% of their total available soluble carbohydrates after 6–8 h. The loss of K⁺ and PO ₄ ^3- followed a similar pattern to that shown by the carbohydrates. Up to 9 daily freeze/thaw cycles gave no increase in metabolite loss for senescent leaves. GLC analysis showed sucrose to be the principal leachate from Acaena. Sucrose, glucose and fructose were the main leachates from Poa. A significant proportion of the soluble carbohydrates in standing dead leaves was trehalose. The relationship of such leachates to microbial decomposition is discussed.     

Interruption of the rat circadian clock by short light-dark cycles

Ninety male Sprague-Dawley rats were exposed to 1:1-h light-dark (LD1:1) cycles for 50-90 days, and then they were released into constant darkness (DD). During LD1:1 cycles, behavioral rhythms were gradually disintegrated, and circadian rhythms of locomotor activity, drinking, and urine 6-sulfatoxymelatonin excretion were eventually abolished. After release into DD, 44 (49%) rats showed arrhythmic behavior for >10 days. Seven (8%) animals that remained arrhythmic for >50 days in DD were exposed to brief light pulses or 12:12-h light-dark cycles, and then they restored their circadian rhythms. These results indicate that the circadian clock was stopped, at least functionally, by LD1:1 cycles and was restarted by subsequent light stimulation.     

Effects of alternating 45-min light-dark cycles on sleep in the rat

We studied the effect of alternating 45-min light-dark (L-D) cycles on sleep in rats. Introduction of short L-D cycles did not abolish the normal circadian rhythm of sleep-wake activity. The amount of non-REM sleep was however increased in the L and decreased in the D 45-min periods. REM was promoted in the D and inhibited in the L 45-min periods. The influence of L-D or D-L transitions depended on the sleep wake activity immediately before the transition.     

Effects of light-dark cycles on the circadian conidiation rhythm inNeurospora crassa

The effects of 24 hr light-dark cycles on the circadian conidiation rhythm inNeurospora crassa were compared among will-typefrq ⁺ and clock mutantsfrq ⁺,frq ³,frq ⁷,frq ⁹ andfrq ¹¹. The minimum length of the light period necessary for complete entrainment to the light-dark cycles was almost 2 hr infrq ⁺,frq ³ andfrq ⁷ strains. The minimum duration of the dark period necessary for the appearance of circadian conidiation was almost 4 hr in all of the strains except thefrq ¹¹ strain. The phase of the conidiation rhythm was dependent on the light to dark transition in thefrq ¹ strain in all light-dark cycles examined and in thefrq ⁺ andfrq ³ strains when the light period was shorter than 16 hr. In contrast, the phase of thefrq ⁷ strain was dependent on the light to dark transition when the light period was shorter than 10 hr.     

Seasonal diurnal acid rhythms in two aquatic crassulacean acid metabolism plants

Summary The diurnal change in titrable acidity in two aquatic CAM plants, Littorella uniflora var. americana (Fern.) Gl. and Isoetes macrospora Duriev., growing at two sites, was monitored at biweekly intervals for two years during the ice-free period. Both plants exhibited the classic pattern of CAM acitivity, with deacidification 60 to 90% complete by noon. The maximum diurnal acid rhythms observed were 169±7, and 154±20 eq·g^-1 fresh weight for the two Littorella populations, and 182±9 and 133±16 eq·g^-1 fresh weight for the two Isoetes populations. The seasonal pattern of the diurnal acid rhythm was correlated with temperature and light. The maximum activity occurred in midsummer, with negligible activity under ice cover. This pattern was similar to that of terrestrial CAM plants from non-arid environments. Comparison of CAM activity for populations of the same species indicates that the magnitude of CAM activity is closely related to plant productivity, and appears to be related to light and perhaps CO₂ availability. In these plants, CAM extends the diel period of carbon accumulation and contributes 40 to 50% of the annual carbon gain. The prolonged period of carbon acquisition and effective conservation of respired CO₂ via CAM is of paramount importance in the growth and productivity of these plants in oligotrophic environments.     

Computational design and evolution of the oscillatory response under light-dark cycles

Circadian clocks are biological systems behaving as oscillators even in constant dark conditions. We propose to use a new strategy based on computational design to provide evidence on the origin and evolution of molecular clocks. We design synthetic molecular clocks having a reduced number of genes and some of them showing architectures found in nature. We analyse the response of our models under diverse forcing light-dark (LD) cycles. Our methodology allows us to evolve networks in silico using various selective pressures, which we apply to the analysis of clocks evolved to be either autonomous or phase locked. Our designed networks either have an oscillatory response with the same period as the forcing LD cycle, or they maintain their free-running period. Our methodology will allow analysing the automatic creation of a free-running period under various LD forcing functions and learning new design principles for circadian clocks.     

Culture of the marine phytoplankter,Dunaliella tertiolecta,with light-dark cycles

Summary Dunaliella tertiolecta was grown in continuous culture, maintained by a single daily dilution to a constant cell concentration, with photoperiods of duration 3 to 18 hours. Illumination was provided with filtered tungsten light having a maximum intensity at 580–590 m. Average intensity at the culture surface was maintained at 0.05 cal/cm²xmin. Temperature was regulated at 20° C. Daily cell production and the fraction of cells dividing daily are reported for each of the photoperiods. Cyclic diurnal variations were noted in cell pigment content, cell volume and photosynthetic rate. Corresponding variations in cell carbon/chlorophyll a ratios were small and may be unimportant for the estimation of biomass from chlorophyll a in field work. An equation for calculating daily growth rate in continuous light, from light absorption by cell chlorophyll a, derived previously, was modified to include the influence of providing light in discrete photoperiods.Supported by U. S. Atomic Energy Commission, Contract AT(11-1)-34 Project 108.     

A digital sensor to measure real-time leaf movements and detect abiotic stress in plants

Plant and plant organ movements are the result of a complex integration of endogenous growth and developmental responses, partially controlled by the circadian clock, and external environmental cues. Monitoring of plant motion is typically done by image-based phenotyping techniques with the aid of computer vision algorithms. Here we present a method to measure leaf movements using a digital inertial measurement unit (IMU) sensor. The lightweight sensor is easily attachable to a leaf or plant organ and records angular traits in real-time for two dimensions (pitch and roll) with high resolution (measured sensor oscillations of 0.36 ± 0.53° for pitch and 0.50 ± 0.65° for roll). We were able to record simple movements such as petiole bending, as well as complex lamina motions, in several crops, ranging from tomato to banana. We also assessed growth responses in terms of lettuce rosette expansion and maize seedling stem movements. The IMU sensors are capable of detecting small changes of nutations (i.e. bending movements) in leaves of different ages and in different plant species. In addition, the sensor system can also monitor stress-induced leaf movements. We observed that unfavorable environmental conditions evoke certain leaf movements, such as drastic epinastic responses, as well as subtle fading of the amplitude of nutations. In summary, the presented digital sensor system enables continuous detection of a variety of leaf motions with high precision, and is a low-cost tool in the field of plant phenotyping, with potential applications in early stress detection.

An inertial measurement unit is capable of measuring dynamic and complex plant organ movements in real-time, and is suitable for early abiotic stress detection.     

Aftereffects of low and high temperature pretreatment on leaf resistance, transpiration, and leaf temperature in xanthium

Leaf resistance for water vapor (total diffusion resistance minus boundary layer resistance), transpiration, and leaf temperature were measured in attached leaves of greenhouse-grown Xanthium strumarium L. plants that had been pretreated for 72 hours with high (40 C day, 35 C night), or low (10 C day, 5 C night) air temperatures. Measurements were made in a wind tunnel at light intensity of 1.15 cal cm⁻² min⁻¹, air temperatures between 5 and 45 C, and wind speed of 65 cm sec⁻¹. Leaf resistances in low temperature pretreated plants were higher (8 to 27 sec cm⁻¹) than in controls or high temperature pretreated plants (0.5 to 3 sec cm⁻¹) at leaf temperatures between 5 and 25 C. Thus, the pretreatment influenced stomatal aperture.     

Suppression of circadian rhythms of pineal and testicular hormones following lithium treatment in normal and reversed light-dark cycles, constant light and constant dark in rats

The aim of the current investigation was to study the effect of lithium on circadian rhythms of pineal - testicular hormones by quantitations of pineal and serum serotonin, N-acetylserotonin and melatonin, and serum testosterone at four time points (06.00, 12.00, 18.00 and 24.00) of a 24-hr period under normal photoperiod (L:D), reversed photoperiod (D:L), constant light (L:L) and constant dark phase (D:D) in rats. Circadian rhythms were observed in pineal hormones in all the combinations of photoperiodic regimens, except in constant light, and in testosterone levels in all the photoperiodic combinations. Pineal and serum N-acetylserotonin and melatonin levels were higher than serotonin at night (24.00 hr), in natural L:D cycle, in reversed L:D cycle or similar to normal L:D cycle in constant dark phase, without any change in constant light. In contrast, testosterone level was higher in light phase (12.00 hr through 18.00 hr) than in the dark phase (24.00 hr through 06.00 hr) in normal L:D cycle, in reversed L:D cycle, similar to normal L:D cycle in constant dark (D:D), and reversed to that of the normal L:D cycle in constant light (L:L). Lithium treatment (2 mEq/kg body weight daily for 15 days) suppressed the magnitude of circadian rhythms of pineal and serum serotonin, N-acetylserotonin and melatonin, and testosterone levels by decreasing their levels at four time points of a 24-hr period in natural L:D or reversed D:L cycle and in constant dark (D:D). Pineal indoleamine levels were reduced after lithium treatment even in constant light (L:L). Moreover, lithium abolished the melatonin rhythms in rats exposed to normal (L:D) and reversed L:D (D:L) cycles, and sustained the rhythms in constant dark. But testosterone rhythm was abolished after lithium treatment in normal (L:D)/reversed L:D (D:L) cycle or even in constant light/dark. The findings indicate that the circadian rhythm exists in pineal hormones in alternate light - dark cycle (L:D/D:L) and in constant dark (D:D), but was absent in constant light phase (L:L) in rats. Lithium not only suppresses the circadian rhythms of pineal hormones, but abolishes the pineal melatonin rhythm only in alternate light - dark cycles, but sustains it in constant dark. The testosterone rhythm is abolished after lithium treatment in alternate light - dark cycle and constant light/dark. It is suggested that (a) normal circadian rhythms of pineal hormones are regulated by pulse dark phase in normal rats, (b) lithium abolishes pineal hormonal rhythm only in pulse light but sustains it in constant dark phase, and (c) circadian testosterone rhythm occurs in both pulse light or pulse dark phase in normal rats, and lithium abolishes the rhythm in all the combinations of the photoperiod. The differential responses of circadian rhythms of pineal and testicular hormones to pulse light or pulse dark in normal and lithium recipients are discussed.     

Nitrogenase activity in the non-heterocystous cyanobacterium Oscillatoria sp. grown under alternating light-dark cycles

The non-heterocystous cyanobacterium Oscillatoria sp. strain 23 fixes nitrogen under aerobic conditions. If nitrate-grown cultures were transferred to a medium free of combined nitrogen, nitrogenase was induced within about 1 day. The acetylene reduction showed a diurnal variation under conditions of continuous light. Maximum rates of acetylene reduction steadily increased during 8 successive days. When grown under alternating light-dark cycles, Oscillatoria sp. fixes nitrogen preferably in the dark period. For dark periods longer than 8 h, nitrogenase activity is only present during the dark period. For dark periods of 8 h and less, however, nitrogenase activity appears before the beginning of the dark period. This is most pronounced in cultures grown in a 20 h light – 4 h dark cycle. In that case, nitrogenase activity appears 3–4 h before the beginning of the dark period. According to the light-dark regime applied, nitrogenase activity was observed during 8–11 h. Oscillatoria sp. grown under 16 h light and 8 h dark cycle, also induced nitrogenase at the usual point of time, when suddenly transferred to conditions of continuous light. The activity appeared exactly at the point of time where the dark period used to begin. No nitrogenase activity was observed when chloramphenicol was added to the cultures 3 h before the onset of the dark period. This observation indicated that for each cycle, de novo nitrogenase synthesis is necessary.     

Circadian rhythms of body temperature and motor activity in rodents their relationships with the light-dark cycle

In rodents, the alternation of light and dark is the main synchronizer of circadian rhythms. The entrainment abilities of the LD cycle could be estimated by experimental modifications of the photoperiod and by following the subsequent temporal distribution of a circadian rhythm. The rate of reentrainment of a rhythm is determined by the nature of the studied variable, by the direction (advance or delay) and the magnitude (or value) of the phase shift. In rodents, core body temperature and motor activity are known to be well synchronized with each other under L:D 12:12 and under constant conditions (LL or DD). There are clear evidences that the circadian pattern of motor activity is generated by two oscillators, one from dusk signal and the other from dawn signal. Whether the circadian rhythms of body temperature and motor activity are generated by a common circadian mechanism or controlled by separate ones still remains unknown. The purpose of this review is to summarize the results obtained on the circadian rhythms of body temperature and motor activity throughout the daily cycle in order to clarify the relationships between these two functions.     

The free-running circadian rhythms of two schizophrenics

Two chronic schizophrenic patients and a psychiatrist spent 21 days in an isolation unit. For the first 4 days they lived on normal time but thereafter the clock was removed and they were free-running. The psychiatrist followed the schedule set by the schizophrenics, one of whom spontaneously decided the times of retiring and rising while the other followed passively. The psychiatrist commonly retired some time later but without disturbing the schizophrenics, the mean duration of whose days was 23.7 h, distinctly shorter than is usual in healthy subjects. This was made up of an activity period of 11.77 h and a rest period of 11.94 h. Pulse rate and temperature were measured frequently throughout the waking hours and rectal temperature was monitored during sleep. Urine samples were also collected throughout the 24 h and were analysed for potassium, sodium, chloride, creatinine, phosphate, calcium and uric acid. Urinary and temperature rhythms followed approximately the activity rhythm, in both healthy and schizophrenic subjects. Pulse rate in the schizophrenics followed a rhythm with a period slightly less than that of activity, and in one schizophrenic showed a consistently early phasing. 11-hydroxycorticosteriods at the end of the observation showed a very early phasing corresponding to that of activity. The findings suggest that schizophrenics may have an abnormally short circadian period.     

Scheduled feeding versus the light-dark cycle on the rhythms of circulating tryptophan, serotonin and N-acetylserotonin in rats

Both the environmental light-dark cycle and scheduled feeding can act as entrainers of biological rhythms. The present study investigated the relative potency of these two environmental cues in entraining the rhythms of circulating tryptophan (TRP), serotonin (5HT) and N-acetylserotonin (NAS). Four groups of rats were subjected for one month to an identical light-dark cycle of 14 h light and 10 h dark with food availability restricted to the 3 h period beginning 2 h after onset of light or onset of darkness. Two groups of animals were food deprived on the day of experiment. The 24 h rhythms of serum TRP, 5HT and NAS were determined. Serum TRP showed a sharp increase after food presentation and declined gradually to a trough just before feeding. Withholding food on the day of experiment abolished this increase. The trough of serum 5HT occurred just before feeding, increased gradually after feeding and peaked 10-13 h afterwards. Serum NAS levels, however, demonstrated an anticipatory rise before feeding, which peaked during feeding and declined to a trough 8 h afterwards. Unlike TRP, withholding food had no effect on either the 5HT or the NAS rhythm. These results indicated that feeding schedule was the common and stronger entrainer for the rhythms of serum TRP, 5HT and NAS. However, each indole had a different rhythm pattern in relation to the feeding schedule which could not be explained by a simple precursor-product relationship.     

Timely administration of melatonin accelerates reentrainment to phase-shifted light-dark cycles in the field mouse Mus booduga

The effect of melatonin on the rate of reentrainment after a 6h phase delay and a 6h phase advance in the light-dark (LD) cycle was assayed in the nocturnal field mouse Mus booduga. After a phase delay of 6h in the LD cycle, a single dose of melatonin (1 mg/kg) was administered for three consecutive days at about CT4 (circadian time 4). After a phase advance of 6h in the LD cycle, melatonin was administered for three consecutive days at about CT22. Melatonin was found to accelerate reentrainment in both cases. Melatonin-treated animals took significantly fewer cycles to reentrain compared to vehicle-treated (50% dimethylsulfoxide [DMSO]) and nontreated control animals.