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Sleep deprivation (SD) is suggested to be associated with reduced thermo-regulatory functions. This study aimed to quantify the effect of partial (PSD) and total (TSD) 24 h SD using a standard heat tolerance test (HTT). Eleven participants underwent HTT after well-rested state, PSD and TSD. No significant physiological differences were found between the exposures but subjective discomfort was higher after TSD. Evening chronotypes' temperature during HTT was higher after TSD compared with PSD (p = 0.017). After TSD, evening chronotypes compared to intermediate chronotypes' temperature was higher during the first hour of the HTT (p < 0.05), suggesting that thermo-regulatory function during exercise in the heat is influenced by chronotype.

The well-known Two-Process Model of Sleep Regulation describes the integration of the circadian rhythm of arousal and sleep – Process C, and the homeostatic pressure to sleep – Process S. Presently, the known biological markers for Process C are melatonin and core body temperature; whereas, for Process S, there is no biological marker except that of aspects of the electroencephalogram (EEG). Endozepines are a class of endogenous compounds that act like benzodiazepines (BZ), i.e., serving as ligands for the BZ binding sites on GABAA receptors. Not much is known about the role of endozepines, in particular non-peptide endozepines, in the sleep field except very few reports about high concentrations observed in endozepine stupor, a rare phenomenon of idiopathic recurring stupor. We focused on hypoxanthine and thromboxane A2, which are considered to have endozepine function. This study aimed to examine the effect of 24 h of acute sleep deprivation on blood levels of hypoxanthine and thromboxane A2 of healthy subjects without sleep problems or disorders. The results showed a significant decrease of both compounds in the morning after sleep deprivation in comparison to the unrestricted normal sleep condition, thereby suggesting that these endozepines are secreted regularly while asleep, and, thus, are necessary for the sleep process. This study is the first to suggest a connection between specific biological markers – endozepines and Process S – in the Two-Process Model of Sleep Regulation and, furthermore, it sheds light on the possible role of endozepines in sleepiness and fatigue. [ABSTRACT FROM AUTHOR]

Cognitive performance at night exhibits a substantial drop, typically before dawn. One of the means of dealing with this phenomenon, as well as with the accompanying sleepiness during sustained wakefulness, is the administration of stimulants. The most widely used and well‐documented stimulants are caffeine, amphetamines, and modafinil. Of these, amphetamines are the least recommended, as they may severely affect behavior. Caffeine and modafinil seem to produce relatively milder side effects and usually only at high doses. Previous comparison studies have revealed equal efficacy of both the stimulants in maintaining alertness and performance during sustained wakefulness. However, these studies used relatively high, and thus not completely safe, doses of these drugs (600 mg caffeine and 400 mg modafinil). Therefore, the aim of the present study was to assess the efficacy of a low and medically safe dose of caffeine (200 mg) and modafinil (200 mg) in maintaining cognitive performance during sustained wakefulness. A flight simulation task was chosen for the assessment of the stimulants in a counter‐balanced, within‐subject design under four different conditions: baseline (no drugs), placebo, caffeine (200 mg), and modafinil (200 mg). The equal effectiveness of both drugs in abolishing the nocturnal drop in cognitive performance, as well as of oral temperature and blood pressure, supported the use of low doses of caffeine and modafinil for the maintenance of alertness in healthy subjects during sustained wakefulness. [ABSTRACT FROM AUTHOR]

PURPOSE. To determine whether parameters calculated from pupillary activity can identify subjects with sleep deprivation, and whether the objective values correlate with a subjective feeling of fatigue. METHODS. pupil size in the dark was recorded continuously for 10 minutes in 12 healthy volunteers using an infrared video camera. Two recordings were made for each subject: after a full night's sleep, and after 24 hours of sleep deprivation. Several parameters calculated from pupil size and activity were analyzed and compared with a subjective rating of the state of alertness provided by the participants in each test. RESULTS. All pupillary parameters differed significantly between alertness and fatigue (p = 0.0076–0.0186). Changes in one of the parameters – average pupillary diameter – correlated with changes in the subjective level of sleepiness (r = -0.51, p = 0.028). Although the values of most parameters differed among subjects, an absolute value of more than 25 in one parameter, cumulative pupillary variability ratio, was always associated with sleep deprivation. CONCLUSION. On-line analysis of the pupillogram using the suggested parameters can be performed easily to produce a real-time assessment of an individual's state of alertness or fatigue that correlates with his/her subjective assessment of this state. [ABSTRACT FROM AUTHOR]