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The criptochrome genes (Cry1 and Cry2) are involved in the molecular mechanism that controls the circadian clock, and mice lacking these genes ([Cry1.sup.-/-]/ [Cry2.sup.-/-]) are behaviorally arrhythmic. It has been speculated that the circadian clock modulates the characteristics of thermoregulation, resulting in body temperature ([T.sub.b]) rhythm. However, there is no direct evidence proving this speculation. We show here that Tb and heat production in [Cry1.sup.-/-]/[Cry2.sup.-/-] mice are arrhythmic under constant darkness. In contrast, both rhythms occur under a light-dark cycle and/or periodical food restriction linked with spontaneous activity and/or eating, although they are not robust as those in wild-type mice. The relationship between heat production and [T.sub.b] in [Cry1.sup.-/-]/[Cry2.sup.-/-] mice is linear and identical under any conditions, indicating that their [T.sub.b] rhythm is determined by heat production rhythm associated with activity and eating. However, [T.sub.b] in wild-type mice is maintained at a relatively higher level in the active phase than the inactive phase regardless of the heat production level. These results indicate that the thermoregulatory responses are modulated according to the circadian phase, and the Cry genes are involved in this mechanism. circadian clock; thermoregulation; metabolism

Jetlag results when a temporary loss of circadian entrainment alters phase relationships among internal rhythms and between an organism and the outside world. After a large shift in the light-dark (LD) cycle, rapid recovery of entrainment minimizes the negative effects of internal circadian disorganization. There is evidence in the existing literature for an activation of the hypothalamic-pituitary-adrenal (HPA) axis after a photic phase shift, and it is possible that the degree of HPA-axis response is a determining factor of reentrainment time. This study utilized a diurnal rodent, Octodon degus, to test the prediction that the alteration of cortisol levels would affect the reentrainment rate of circadian locomotor rhythms. In experiment 1, we examined the effects of decreased cortisol (using metyrapone, an 11[beta]-hydroxylase inhibitor) on the rate of running-wheel rhythm recovery after a 6-h photic phase advance. Metyrapone treatment significantly shortened the length of time it took animals to entrain to the new LD cycle (11.5% acceleration). In experiment 2, we examined the effects of increased cortisol on the rate of reentrainment after a 6-h photic phase advance. Increasing plasma cortisol levels increased the number of days (8%) animals took to reentrain running-wheel activity rhythms, but this effect did not reach significance. A third experiment replicated the results of experiment 1 and also demonstrated that suppression of HPA activity via dexamethasone injection is capable of accelerating reentrainment rates by ~33%. These studies provide support for an interaction between the stress axis and circadian rhythms in determining the rate of recovery from a phase shift of the LD cycle. stress; circadian rhythm; metyrapone; dexamethasone; entrainment

The present study aimed to identify the hypothalamic nuclei involved with food entrainment by using c-Fos-like immunoreactivity (c-Fos-IR) as a marker of functional activation. We studied rats entrained 3 wk to restricted feeding schedules (RF), their ad libitum (AL) controls, and the persistence of c-Fos-IR temporal patterns in entrained-fasted rats. In addition, we included 22-h fasting and 22-h fasting-refeeding groups as controls of fasting and refeeding acute effects. Diurnal patterns of c-Fos-IR were observed in the tuberomammilar nucleus (TM) and suprachiasmatic nucleus (SCN) in AL rats. In all nuclei, except the SCN and ventromedial nucleus (VMH), restricted feeding schedules imposed a temporal pattern of increased c-Fos-IR around mealtime. An increase in c-Fos-IR before and after meal time was observed in dorsomedial nucleus (DMH), lateral nucleus (LH), perifornical area (PeF), and TM, and a marked increase was observed in the paraventricular nucleus (PVN) after feeding. Food-entrained c-Fos-IR patterns persisted after 3 days in fasting in DMH, LH, and PeF. Present data suggest that FEO might not rely on a single nucleus and rather may be a distributed system constituted of interacting nuclei in which the PVN is mainly involved with the response to signals elicited by food ingestion and, therefore, with the entraining pathway. We can suggest that the PeF and TM may be involved with the arousal state during food anticipation and the DMH and LH with the time-keeping mechanism of FEO or its output. circadian rhythms; entrainment; food-entrained oscillator; food anticipatory activity

Circadian rhythms in neuronal ensemble, subpopulations, and single unit activity were recorded in the suprachiasmatic nuclei (SCN) of rat hypothalamic slices. Decomposition of the ensemble pattern revealed that neuronal subpopulations and single units within the SCN show surprisingly short periods of enhanced electrical activity of [approximately equal to] 5 h and show maximal activity at different phases of the circadian cycle. The summed activity accounts for the neuronal ensemble pattern of the SCN, indicating that circadian waveform of electrical activity is a composed tissue property. The recorded single unit activity pattern was used to simulate the responsiveness of SCN neurons to different photoperiods. We inferred predictions on changes in peak width, amplitude, and peak time in the multiunit activity pattern and confirmed these predictions with hypothalamic slices from animals that had been kept in a short or long photoperiod. We propose that the animals' ability to code for day length derives from plasticity in the neuronal network of oscillating SCN neurons.

Experimental data on the circadian ([approximately equal to] 24-h) clock in mammalian cells are vast, diverse, and detailed. Mathematical models are therefore needed to piece these data together and to study overall clock behavior. Previous models have focused on Neurospora or Drosophila or can be converted to a Drosophila model simply by renaming variables. Those models used Hill-type terms for transcription regulation and Michaelis-Menten type or delay terms for posttranslation regulation. Recent mammalian experimental data call into question some of the assumptions in these approaches. Moreover, gene duplication has led to more proteins in the mammalian system than in lower organisms. Here we develop a detailed distinctly mammalian model by using mass action kinetics. Parameters for our model are found from experimental data by using a coordinate search method. The model accurately predicts the phase of entrainment, amplitude of oscillation, and shape of time profiles of clock mRNAs and proteins and is also robust to parameter changes and mutations. mathematical models | eukaryotic transcription regulation | PER | CRY | CLOCK

The circadian clock in mammals is located in the hypothalamic suprachiasmatic nucleus. At the core of the clock are molecular autofeedback loops associated with clock gene transcription. However, the mechanisms of circadian signal transduction are basically unknown. A recent report by Ikeda et al. provides new insights into the intracellular signaling pathways involved in conveying circadian clock information from the core loop to cellular functions. Cytosolic [Ca.sup.2+] is proposed to be a key substance linking the 'pendulum' to the 'hands' of the clock.

The MAP kinase cascade and its possible role in resetting the mammalian circadian clock are discussed in this research communication. The suprachiasmatic nucleus (SCN) is the primary timing center in mammals, but peripheral/ immortalized cells have a circadian clock as well. Induction of the circadian oscillation of gene expression is started by TPA treatment of NIH-3T3 fibroblasts. NIH-3T3 is inhibited by a MEK inhibitor. Prolonged activation of the MAPK cascade is sufficient to bring on circadian gene expression.

We validated the use of urine to monitor changes in the activity of both the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS) in swine. Ten pregnant sows were fitted with venous catheters 3 wk after mating. In the early (wk 6), middle (wk 9), and late (wk 14) stages of gestation, blood and urine were collected over 24 h to monitor diurnal changes in plasma cortisol, urinary cortisol, and urinary catecholamines (norepinephrine [NE] and epinephrine [EPI]). Dexamethasone suppression tests (DST) and ovine corticotropin-releasing hormone (CRH) challenge tests were also performed at each stage of gestation. All plasma and urinary values changed markedly around the clock. Diurnal variations of urinary cortisol were comparable to those in plasma, with a late nocturnal peak and a trough occurring in the evening. During the dark period, urinary catecholamines were lower than during the light period. Norepinephrine increased sharply after lights came on and peaked after meal time. Epinephrine began to rise at the end of the dark period and peaked just before meal time. Average plasma cortisol increased with the stage of gestation, due to higher levels during daylight hours. Dexamethasone at 2000 (20 (mu)g/kg i.v.) decreased plasma cortisol at 0830 and nocturnal cortisol excretion. The magnitude of the decrease in plasma ACTH and urinary cortisol after DST was lower in late than in early and midgestation, indicating increased feedback resistance at that stage. The CRH (1 (mu)g/kg i.v.) increased plasma and urinary cortisol. Peak levels occurred 30 min and 2 to 3 h after the injection, respectively. Catecholamines and cortisol in urine produced during the night (2000 to 0800) and the early morning (0400 to 0800 and 0800 to 0900) were highly correlated with their 24-h excretion rate. These results indicate that it is possible to monitor changes in the HPA axis and SNS activity through urinary measurements in pigs. Key Words: Hydrocortisone, Catecholamines, Urine, Pigs, Circadian Rhythm, Pregnancy

The role of glucose availability in circadian responses to light in mice has been investigated. To this end, photic phase resetting of the circadian rhythm of locomotor function in mice exposed to four metabolic challenges was analyzed. There was an observed reduction in phase delays in fed mice pretreated with 5 IU/kg sc insulin and in fasted samples injected with saline or insulin compared with control fed mice. These findings demonstrate that circadian responses to light are inhibited when brain glucose availability is reduced, an indication of a metabolic modulation of light-induced phase shifts.

Research indicates that circadian activity rhythm phase shifts brought on by melatonin administration are largely unaffected by a reduction in melatonin receptor genetic expression in the suprachiasmatic nucleus of mice. It seems that the circadian time system exhibits a constant reaction to melatonin, irrespective of age. Experiments using 2-[125I]iodomelatonin binding techniques are described.

Adaptation to a changing environment is an essential feature of physiological regulation. The day-night rhythm is translated into hormonal oscillations governing the metabolism of all living organisms. In mammals the pineal gland is responsible for the synthesis of the hormone melatonin in response to signals originating from the endogenous clock located in the hypothalamic suprachiasmatic nucleus (SCN). The molecular mechanisms involved in rhythmic synthesis of melatonin involve the cAMP response element modulator (crem) gene, which encodes transcription factors responsive to activation of the cAMP signalling pathway. The CREM product, inducible cAMP early repressor (ICER), is rhythmically expressed and participates in a transcriptional autoregulatory loop that also controls the amplitude of oscillations of 5-HT N-acetyl transferase, the rate-limiting enzyme of melatonin synthesis. Thus, a transcription factor modulates the oscillatory levels of a hormone.

Researchers have sought to identify the impact of vitamin B12 (VB12) on the rat suprachiasmatic nucleus (SCN) by investigating the contents of upsilon-aminobutyric acid (GABA) and glutamate (Glu), its main neurotransmitters. The results indicated that VB12 produces a marked rise in GABA content in the SCN and cerebral cortex. It appears that GABA receptors may be involved in the regulation of the circadian clock system. GABA metabolism seems to be involved in the phase-resetting function as vigabatrin, an inhibitor of GABA catabolism, boosts light-induced phase delays.

Researchers have investigated the possibility that serotonergic afferents may play a role in the activity-dependent nonphotic entrainment mechanism in rodents. This work was carried out through the selective lesioning of serotonin terminals within and adjacent to the suprachiasmatic nuclei (SCN) by local microinjection of 5,7-dihydroxytryptamine in mice pretreated with desipramine. It was concluded that serotonergic afferents to the SCN are required for the transduction of a volitional activity-dependent entrainment stimulus.

Two newly isolated mutant strains of Neurospora crassa, cpz-1 and cpz-2, were hypersensitive to chlorpromazine with respect to mycelial growth but responded differently to the drug with respect to the circadian conidiation rhythm. In the wild type, chlorpromazine caused shortening of the period length of the conidiation rhythm. Pulse treatment with the drug shifted the phase and inhibited light-induced phase shifting in Neurospora. By contrast to the wild type, the cpz-2 strain was resistant to these inhibitory effects of chlorpromazine. Inhibition of cpz-2 function by chlorpromazine affected three different parameters of circadian conidiation rhythm, namely, period length, phase and light-induced phase shifting. These results indicate that the cpz-2 gene must be involved in or related closely to the clock mechanism of Neurospora. By contrast, the cpz-1 strain was hypersensitive to chlorpromazine with respect to the circadian conidiation rhythm.

In the rat suprachiasmatic nucleus slice culture, circadian rhythms in the release of arginine vasopressin and vasoactive intestinal polypeptide were measured simultaneously and longitudinally. The phase relationship between the two peptide rhythms was relatively constantin the culture without a treatment of antimitotic drugs but became diverse by an introduction of antimitotics, which is generally used to reduce the number of glial cells. By monitoring the two rhythms continuously for 6 days, different periods were detected in culture with the antimitotic treatment. Furthermore, N-methyl-D-aspartate shifted the phase of the two peptide rhythms in the same culture differently. These results indicate that the arginine vasopressin and vasoactive intestinal polypeptide release are under control of different circadian oscillators.

The reindeer (Rangifer tarandus tarandus) is a semidomesticated holarctic ruminant subject to economic activity. Our research was carried out in Northern Finland, at latitude 69 [degrees] 10 [minutes] N, where the daily light-dark rhythm has a polar pattern: constant darkness starts on November 26th and lasts about 10 weeks, while the polar day of about 10 weeks starts on May 15th. We had previously found distinct daily rhythms in reindeer melatonin secretion in autumn, winter and spring but not at all in summer. Here we determine how the daily melatonin pattern develops after the period of polar day and how it disappears at the beginning of that period. Melatonin showed increased concentrations very soon after the first sunset. Thereafter the daily pattern developed gradually and was fully developed around the autumn equinox. Its shape was asymmetric and the maximum amplitude occurred near the end of the dark period. The disappearance of this daily pattern in spring was also gradual. The maximal concentrations depended on ambient illumination, especially the duration of the dark period. In spring the reindeer appeared to be more sensitive to light than in autumn. Additionally illumination of the same magnitude suppressed serum melatonin levels more effectively before midnight than afterwards, which is in accordance with the asymmetric secretion pattern. Melatonin secretion was always suppressed when the ambient illumination exceeded 1,000 lx. The pattern of melatonin secretion in reindeer is different from those described earlier and hence the reindeer may serve as a model for melatonin secretion at high latitudes. We suggest that, in addition to the duration of the melatonin secretion, changes in amplitude are also of importance in the reindeer.

A study reveals that phytochrome and a circadian clock, regulate the expression of genes encoding the light, harvesting chlorophyll a/b binding proteins of photosystem II (Cab) in etiolated wheat seedings. The expression of the cab genes, especially that of the cab-1 gene in circadian manner, can be stimulated by the photoconversion of < 1% of phytochrome to its active form, which can be attained by moonlight. A very low and a low fluence response can be seen in this reaction. The circadian oscillation of the Cab-1 mRNA level will persist for a minimum of 100 hours with only one far-red light pulse given to an etiolated seedling.

The hypothesis that the biological clock is based on proteins and the events regulating protein synthesis was investigated. The study protocol involved the treatment of the ocular circadian pacemaker of the the mollusc Bulla gouldiana with the protein synthesis inhibitors anisomycin and cycloheximide. The results showed that the free-running period of the circadianrhythm was significantly lenghtened. Furthermore, long pulse treatments of various lengths affected the rhythm in a phase-dependent manner. These results indicate a role for protein synthesis in the regulation of the circadian pacemaker.

The combined effects of ethanol and circadian sleepiness/alertness levels were examined using the sedative effect of ethanol. It was found that a moderate amount of ethanol substantially increases physiological sleepiness in the early morning hours, even in persons who are usually alert during these periods. The study thus concludes that the ingestion of ethanol brings down a person's alertness level and contributes to related performance deficits that usually occur at night.

The crustacean hyperglycemic hormone (CHH) is involved in the regulation of endogenous blood glucose metabolism. In this paper we describe the daily rhythmicity in the blood glucose and the blood CHH content of the crayfish Orconectes limosus. Both blood CHH and blood glucose levels increase during the first hours after the beginning of darkness. The bioactivity of released CHH is far higher than that of CHH stored in the sinus gland. Moreover, the released hyperglycemic material shows an affinity for high molecular weight proteins in the hemolymph. Preliminary results suggest that subunits of hemocyanin may act as potential carrier-proteins for bioactive CHH.

The primary hallmark of circadian clocks is their ability to entrain to environmental stimuli. The dominant, and therefore most physiologically important, entraining stimulus comes from environmental light cycles. Here we describe the establishment and characterization of a new cell line, designated Z3, which derives from zebrafish embryos and contains an independent, light-entrainable circadian oscillator. Using this system, we show distinct and differential light-dependent gene activation for several central clock components. In particular, activation of Per2 expression is shown to be strictly regulated and dependent on light. Furthermore, we demonstrate that Per1, Per2, and Per3 all have distinct responses to light-dark (LD) cycles and light-pulse treatments. We also show that Clock, Bmal1, and Bmal2 all oscillate under LD and dark-dark conditions with similar kinetics, but only Clock is significantly induced while initiating a light-induced circadian oscillation in Z3 cells that have never been exposed to a LD cycle. Finally, our results suggest that Per2 is responsible for establishing the phase of a circadian rhythm entraining to an alternate LD cycle. These findings not only underscore the complexity by which central clock genes are regulated, but also establishes the Z3 cells as an invaluable system for investigating the links between light-dependent gene activation and the signaling pathways responsible for vertebrate circadian rhythms.

Activation of [MT.sub.2] melatonin receptors in rat suprachiasmatic nucleus phase advances the circadian clock. Am J Physiol Cell Physiol 280: C110-C118, 2001.--The aim of this study was to identify the melatonin receptor type(s) ([MT.sub.1] or [MT.sub.2]) mediating circadian clock resetting by melatonin in the mammalian suprachiasmatic nucleus (SCN). Quantitative receptor autoradiography with 2-[[sup.125]I]iodomelatonin and in situ hybridization histochemistry, with either [sup.33]P- or digoxigenin-labeled antisense [MT.sub.1] and [MT.sub.2] melatonin receptor mRNA oligonucleotide probes, revealed specific expression of both melatonin receptor types in the SCN of inbred Long-Evans rats. The melatonin receptor type mediating phase advances of the circadian rhythm of neuronal firing rate in the SCN slice was assessed using competitive melatonin receptor antagonists, the [MT.sub.1]/[MT.sub.2] nonselective luzindole and the [MT.sub.2]-selective 4-phenyl-2-propionamidotetraline (4P-PDOT). Luzindole and 4P-PDOT (1 nM-1 [micro]M) did not affect circadian phase on their own; however, they blocked both the phase advances (~4 h) in the neuronal firing rate induced by melatonin (3 pM) at temporally distinct times of day [i.e., subjective dusk, circadian time (CT) 10; and dawn, CT 23], as well as the associated increases in protein kinase C activity. We conclude that melatonin mediates phase advances of the SCN circadian clock at both dusk and dawn via activation of [MT.sub.2] melatonin receptor signaling. suprachiasmatic nucleus; brain slice electrophysiology; luzindole; 4-phenyl-2-propionamidotetraline; protein kinase C

Adaptation of circadian rhythms to the environmental light-dark cycle is necessary for the survival of organisms. This synchronization or entrainment is only caused by light (photic input) during the dark period, according to the internal biological clock of an organism. During the light period, internal factors (non-photic input), rather than light, are able to entrain circadian rhythms. In this article, the data that implicate the neurotransmitter system for histamine in circadian entrainment are reviewed. Furthermore, we speculate that histamine receptors are the final gate at which both photic and non-photic entrainment mechanisms converge before sending a resetting signal to the intracellular biological clock.

Ruby, Norman F., Margarita L. Dubocovich, and H. Craig Heller. Siberian hamsters that fail to reentrain to the photocycle have suppressed melatonin levels. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 278: R757-R762, 2000.--Siberian hamsters readily reentrain to a 3-h phase delay of the photocycle (16 h light/day) but fail to reentrain to a 5-h phase delay. This study tested whether melatonin production was suppressed in animals that failed to reentrain. Melatonin was measured on the day before, day of, or several days after each phase shift. Melatonin levels measured 4 h after dark onset were ~83 [micro]g/ml on the day before each phase delay and undetectable ([is less than] 6 [micro]g/ml) during the light phase on the day of the phase shift. Activity onsets regained their prior phase relationship to the photocycle 4 (3 h) or 5 (5 h) days after the phase shift; on that day, melatonin levels were measured 4 h after dark onset. Melatonin levels were unaffected by the 3-h phase delay ([is greater than] 57.6 [micro]g/ml) but were undetectable after a 5-h phase delay ([is less than] 8 [micro]g/ml). Thus melatonin remained suppressed only after the phase delay to which hamsters also fail to reentrain. This relationship suggests that the propensity for reentrainment maybe influenced by changes in melatonin production following a phase shift of the photocycle. circadian system; light-dark cycle; activity; phase shift

A negative feedback control of kaiC expression by KaiC protein has been proposed to generate a basic oscillation of the circadian clock in the cyanobacterium Synechococcus sp. PCC 7942. KaiC has two P loops or Walker's motif As, that are potential ATP-/GTP-binding motifs and DXXG motifs conserved in various GTP-binding proteins. Herein, we demonstrate that in vitro KaiC binds ATP and, with lower affinity, GTP. Point mutation by site-directed mutagenesis of P loop 1 completely nullified the circadian rhythm of kaiBC expression and markedly reduced ATP-binding activity. Moreover, KaiC can be autophosphorylated in vitro. These results suggest that the nucleotide-binding activity of KaiC plays important roles in the generation of circadian oscillation in cyanobacteria.

Aeschbach, Daniel, Jeffery R. Matthews, Teodor T. Postolache, Michael A. Jackson, Holly A. Giesen, and Thomas A. Wehr. Two circadian rhythms in the human electroencephalogram during wakefulness. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R1771-R1779, 1999.--The influence of the circadian pacemaker and of the duration of time awake on the electroencephalogram (EEG) was investigated in 19 humans during ~40 h of sustained wakefulness. Two circadian rhythms in spectral power density were educed. The first rhythm was centered in the theta band (4.25-8.0 Hz) and exhibited a minimum ~1 h after the onset of melatonin secretion. The second rhythm was centered in the high-frequency alpha band (10.25-13.0 Hz) and exhibited a minimum close to the body temperature minimum. The latter rhythm showed a close temporal association with the rhythms in subjective alertness, plasma melatonin, and body temperature. In addition, increasing time awake was associated with an increase of power density in the 0.25- to 9.0-Hz and 13.25- to 20.0-Hz ranges. It is concluded that the waking EEG undergoes changes that can be attributed to circadian and homeostatic (i.e., sleep-wake dependent) processes. The distinct circadian variations of EEG activity in the theta band and in the high-frequency alpha band may represent electrophysiological correlates of different aspects of the circadian rhythm in arousal. homeostatic process; alertness; melatonin; body temperature; evening wake maintenance

The circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus organizes behavioral rhythms, such as the sleep-wake cycle, on a near 24-h time base and synchronizes them to environmental day and night. Light information is transmitted to the SCN by direct retinal projections via the retinohypothalamic tract (RHT). Both glutamate (Glu) and pituitary adenylyl cyclase-activating peptide (PACAP) are localized within the RHT. Whereas Glu is an established mediator of light entrainment, the role of PACAP is unknown. To understand the functional significance of this colocalization, we assessed the effects of nocturnal Glu and PACAP on phasing of the circadian rhythm of neuronal firing in slices of rat SCN. When coadministered, PACAP blocked the phase advance normally induced by Glu during late night. Surprisingly, blocking PACAP neurotransmission, with either PACAP6-38, a specific PACAP receptor antagonist, or anti-PACAP antibodies, augmented the Glu-induced phase advance. Blocking PACAP in vivo also potentiated the light-induced phase advance of the rhythm of hamster wheel-running activity. Conversely, PACAP enhanced the Glu-induced delay in the early night, whereas PACAP6-38 inhibited it. These results reveal that PACAP is a significant component of the Glu-mediated light-entrainment pathway. When Glu activates the system, PACAP receptor-mediated processes can provide gain control that generates graded phase shifts. The relative strengths of the Glu and PACAP signals together may encode the amplitude of adaptive circadian behavioral responses to the natural range of intensities of nocturnal light.

Cajochen, Christian, Sat Bir S. Khalsa, James K. Wyatt, Charles A. Czeisler, and Derk-Jan Dijk. EEG and ocular correlates of circadian melatonin phase and human performance decrements during sleep loss. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R640-R649, 1999.--The aim of this study was to quantify the associations between slow eye movements (SEMs), eye blink rate, waking electroencephalogram (EEG) power density, neurobehavioral performance, and the circadian rhythm of plasma melatonin in a cohort of 10 healthy men during up to 32 h of sustained wakefulness. The time course of neurobehavioral performance was characterized by fairly stable levels throughout the first 16 h of wakefulness followed by deterioration during the phase of melatonin secretion. This deterioration was closely associated with an increase in SEMs. Frontal low-frequency EEG activity (1-7 Hz) exhibited a prominent increase with time awake and little circadian modulation. EEG alpha activity exhibited circadian modulation. The dynamics of SEMs and EEG activity were phase locked to changes in neurobehavioral performance and lagged the plasma melatonin rhythm. The data indicate that frontal areas of the brain are more susceptible to sleep loss than occipital areas. Frontal EEG activity and ocular parameters may be used to monitor and predict changes in neurobehavioral performance associated with sleep loss and circadian misalignment. melatonin; sleepiness; wake maintenance zone; spectral analysis; constant routine; cross-correlation; cognitive performance; electroencephalogram; electrooculogram