Pablo Valdez

Delaying and Extending Sleep During Weekends: Sleep Recovery or Circadian Effect?

There is a well-known tendency to delay and prolong our sleep during weekends (Saturday and Sunday), with an advance and reduction of sleep during workdays (Monday to Friday). The objective of this work was to determine if the changes of sleep during weekends are produced by a partial sleep deprivation or a lack of entraining of circadian rhythms to an advanced phase, during workdays. The subjects were 52 undergraduate female students, mean age = 17.5 years, SD = 1.32. All students attended school following a regular schedule, from Monday to Friday. Two groups of students were studied: one attended school from 07:00 to 12:00 h (morning group, n = 30); the other attended school from 14:00 to 18:00 (afternoon group, n = 22). None of the students worked or was engaged in other activity with a fixed schedule. All kept a sleep-wake diary for 2 weeks, in which they recorded their bedtimes, wakeup times, and sleep-onset latencies. The morning group delayed 47.4 min [t(29) = 4.72, p < 0.0001] and prolonged their sleep 118.2 min [t(29) = 9.4, p < 0.0001] during weekends. Although the afternoon group had the opportunity to maintain a delayed phase and a long sleep time throughout the week, they delayed their bedtime by 24 min [t(21) = 2.99, p < 0.01] during weekends, without changing their sleep duration. The findings suggest that the prolonged sleep during weekends is due to reduction of sleep during workdays, whereas the delay of bedtime seems to be associated with a tendency of the human circadian system to maintain a delayed phase with respect to the solar daylight period.

Circadian rhythms in components of attention

Attention processes involve different components, such as phasic alertness, selective attention and vigilance (sustained attention, concentration). The aim of this study was to identify possible circadian rhythms in these attention components. Eight female undergraduate students (mean age 17.5 yr, SD = 0.93, range 16 – 19 yr) participated voluntarily in this study; they attended classes from 7:00 to 13:30 hours, from Monday to Friday. Each subject was recorded in a constant routine protocol for 30 h, during which rectal temperature was recorded at one-minute intervals. Sleepiness, tiredness and a continuous performance task were assessed each hour. All performance measures showed a decline through the 30-h session. Indicators of tonic alertness, phasic alertness and selective attention showed circadian variations, whereas indicators of vigilance (sustained attention, concentration) did not show circadian variations. Circadian variations in these attention components may be critical for the performance of many tasks, such as memory, reading, arithmetic calculation, etc. Dissociation of vigilance from the other attention components suggests a strong link between this variable and fatigue (homeostatic process). Circadian variations in attention components are also relevant to the decrease of productivity and higher risk of accidents during night shift work.

CIRCADIAN AND HOMEOSTATIC VARIATION IN SUSTAINED ATTENTION

Human performance is modulated by circadian rhythms and homeostatic changes. Changes in efficiency in the performance of many tasks might be produced by variation in a basic cognitive process, such as sustained attention. This cognitive process is the capacity to respond efficiently to the environment during prolonged periods (from minutes to hours). There are three indices of sustained attention: general stability of efficiency, time on task stability, and short-term stability. The objective of this work was to analyze circadian and homeostatic influences on the indices of sustained attention. Participants were nine undergraduate female student volunteers (mean age 17.67 yrs, SD = 1.00, range 16–19 yrs) who attended school from 07:00–13:30 h, Monday to Friday. They were assessed while adhering to a modified 28 h constant-routine protocol during which feeding, room temperature, motor activity, and room illumination were controlled. Rectal temperature was recorded each minute, and indices of sustained attention were assessed hourly through a continuous performance task (CPT). General stability was measured as standard deviation of correct responses and reaction time, time on task stability was measured as the linear regression of correct responses and reaction time throughout the task, and short-term stability was measured as hit runs and error runs. Rectal temperature showed circadian variation; subjective somnolence and tiredness increased, while general performance and all indices of sustained attention declined throughout the 28 h recording session. General stability exhibited circadian variation, whereas time on task did not. Short-term stability showed circadian variations in short-error runs, long-error runs, and short-hit runs, but long-hit runs did not. There was a 26 sec short interval at the beginning of the task, characterized by a very high efficiency level of performance. Execution during this safe period was not affected by time awake and did not show circadian variation. Overall, these results suggest a dissociation of the effects of circadian and homeostatic factors on the indices of sustained attention. General stability, short- and long-error runs, and short-hit runs were modulated by both a homeostatic factor and a circadian effect, while long-hit runs and time on task stability were modulated only by a homeostatic factor and did not show circadian variation. There was also a 26 sec “safe period” that seems to be independent of circadian and homeostatic influences. These results suggest that performance at work deteriorates at the end of a shift due to a decrease in general stability, an increase in error runs, decrease of long-hit runs, and decline with time on task. Night shiftworkers are exposed to an additional deterioration of performance during the nighttime due to a decrease in general stability and an increase in error runs. (Author correspondence: valdez.pa@gmail.com)

Circadian rhythms in phonological and visuospatial storage components of working memory

Abstract Working memory is a basic cognitive process that temporarily maintains the information necessary for the performance of many complex tasks such as reading comprehension, learning and reasoning. Working memory includes two storage components: phonological and visuospatial, and a central executive control. The objective of this study was to identify possible circadian rhythms in phonological and visuospatial storage components of working memory using a constant routine protocol. Participants were eight female undergraduate students, aged 17.5±0.93, range = 16 – 19 years old. They were recorded in the laboratory in a constant routine protocol during 30 h. Rectal temperature was recorded every minute; subjective sleepiness and tiredness, as well as phonological and visuospatial working memory tasks, were assessed each hour. There were circadian variations in correct responses in phonological and visuospatial working memory tasks. Cross-correlation analysis showed a 1-h phase delay of the phonological storage component and a 3-h phase delay of the visuospatial storage component with respect to rectal temperature. This result may explain the changes in the performance of many complex tasks during the day.

Circadian rhythms in cognitive performance: implications for neuropsychological assessment

Circadian variations have been found in human performance, including the efficiency to execute many tasks, such as sensory, motor, reaction time, time estimation, memory, verbal, arithmetic calculations, and simulated driving tasks. Performance increases during the day and decreases during the night. Circadian rhythms have been found in three basic neuropsycho- logical processes (attention, working memory, and executive functions), which may explain oscillations in the performance of many tasks. The time course of circadian rhythms in cognitive performance may be modified significantly in patients with brain disorders, due to chronotype, age, alterations of the circadian rhythm, sleep deprivation, type of disorder, and medication. This review analyzes the recent results on circadian rhythms in cognitive performance, as well as the implications of these rhythms for the neuropsychological assessment of patients with brain disorders such as traumatic head injury, stroke, dementia, developmental disorders, and psychiatric disorders.

Circadian rhythms in two components of executive functions: cognitive inhibition and flexibility

The objective of this study was to analyze possible circadian variations in two components of executive functions: inhibition and flexibility. Participants were eight undergraduate students, age: 17.75 ± 0.46 years, one male and seven female. They were kept in a constant routine protocol for 29 h. Rectal temperature was recorded every minute and responses to a computerized Stroop-like task with shifting criteria were recorded every 100 min. The task had three sections: match, no-match (index of inhibition), and shifting criteria (index of flexibility). There were circadian variations in rectal temperature, inhibition and flexibility. These cognitive processes showed a 1–2 h phase delay with respect to rectal temperature, and were modulated by sustained attention (time on task). The decline of these components of executive functions at night and in the early morning impairs decision-making and problem-solving, thus promoting errors while driving a car or working at night.

Adjustment of the Sleep-Wake Cycle to Small (1-2h) Changes in Schedule

Changes of schedules larger than 3 h, such as jet lag and shift work, require an adjustment period of several days to resynchronize the sleep-wake cycle and several weeks to resynchronize other circadian rhythms to the new schedule. Initial studies on adaptation to small changes of schedule (1-2 h) found that the sleep-wake cycle adapts to the new schedule in less than 48 h, and such modifications are generally not studied because they may be confounded by a potential masking effect. This article summarizes the few published studies on Daylight Saving Time (DST) and sleep during weekends, two examples of small changes in schedule. There are individual differences in adaptation to daylight saving time, while some persons adjust immediately; other persons require more than 2 weeks. During weekends, people tend to go to bed and wake up later, and to extend their sleep. Delay and extension of sleep depend on factors such as shift of work during weekdays and chronotype (morningness-eveningness). Both DST and sleep during weekends offer the opportunity to study adaptation of the sleep-wake cycle in recurrent, social conditions. Studying these phenomena is also relevant to some socioeconomic issues, like the reportedincrease of traffic accidents and complaints from the population during daylight saving time; or the possible decrease in productivity and absenteeism during the ‘Blue Monday’.

Sleep–wake habits and circadian preference in Mexican secondary school

OBJECTIVE The current study aimed to determine the differences between sleep-wake habits and circadian preference in Mexican adolescents attending classes at a morning shift or an afternoon shift. METHODS The sample consisted of 568 students of a secondary school in Reynosa, northeastern Mexico, of whom 280 were boys and 288 were girls (mean age 14.08 ± 0.72 years, age range 13-16 years). In the morning shift, 287 students attend classes on a schedule from 7:30 to 13:00 and the afternoon shift, 281 students, on a schedule from 13:20 to 19:00. Students completed a general information questionnaire, the Sleep Timing Questionnaire and the Spanish version of the Morningness-Eveningness Questionnaire. RESULTS The adolescents who attended the morning shift had earlier bedtime and waking time, but shorter sleep duration than those who attended the afternoon shift. Those oriented to eveningness had later bedtime, waking time, and a shorter sleep duration than those oriented to morningness. Two interactions were found between school shift and chronotype. First, with regard to waking time during weekdays, students who attended the afternoon shift and were oriented to eveningness woke up later than those who attended the morning shift and were oriented to eveningness; during weekdays, there were no differences between the waking time of morning-type and evening-type students who attended the morning shift. Second, with regard to sleep duration on weekdays, students who attended the morning shift and were oriented to eveningness had the shortest sleep duration. Furthermore, there were no differences between sleep duration on weekdays in evening-type and morning-type students of the afternoon shift. CONCLUSION Adolescents who attend classes in the morning shift and are oriented to eveningness are the most sleep deprived. Those who attend the afternoon shift will have optimal sleep duration, regardless of their circadian preference.

Effects of sleep reduction on the phonological and visuospatial components of working memory.

Sleep reduction impairs the performance of many tasks, so it may affect a basic cognitive process, such as working memory, crucial for the execution of a broad range of activities. Working memory has two storage components: a phonological and a visuospatial component. The objective of this study was to analyze the effects of sleep reduction for 5 days on the storage components of working memory. Thirteen undergraduate students (18.77±2.20 years of age), 5 men and 8 women, responded two N-Back tasks (auditory and visual), with three sections each (0-Back, 1-Back, and 2-Back). These tasks were performed at 13:00 h under the following conditions: before sleep reduction (control; C); on the first (SR1), fourth (SR4), and fifth (SR5) days of sleep reduction (4 h of sleep per night); and one day after they slept freely (recovery, R). Sleep reduction produced a decrement in accuracy on the auditory 2-Back section the fifth day of sleep reduction (C=87.86±13.35%; SR5=74.76±16.37%; F=14.57, p<0.01). In the visual 2-Back section accuracy decreased (C=88.10±9.95%; SR1=82.45±11.57%; SR5=77.76±14.14%; F=10.80, p<0.05), and reaction time increased (C=810.02±173.96 ms; SR1=913.51±172.25 ms; SR5=874.78±172.27 ms; F=10.80, p<0.05) on the first and fifth day of sleep reduction. In conclusion, five days of sleep reduction produces a decrease in the phonological and visuospatial storage components of working memory, which may interfere with processing verbal information and solving problems that require spatial analysis.

Circadian variations in self-monitoring, a component of executive functions

The objective of this study was to identify circadian rhythms in self-monitoring, a component of executive functions. Participants were 10 undergraduate students, age: 18.5 ± 2.68 years, two male and eight female. They were recorded on a 30-h constant routine protocol; rectal temperature was recorded every minute and performance on a tracking task was assessed every 100 min. Self-monitoring indicators were adjustments of responses to random changes of speed and trajectory of a circle moving on the computer screen. Participants showed better accuracy during the afternoon, with decreases in the morning (06:20 and 08:00 h). These variations showed a phase delay of 2:29 ± 2:19 h with respect to the circadian rhythm of body temperature. In conclusion, there are circadian variations in self-monitoring. The decline in this component of executive functions could cause serious accidents among people working or studying during a morning shift, as well as commuting to and from work or school.