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It is well established that the overt rhythms of all behavioral variables studied to date, including sleep organization and propensity, subjective alertness, cognitive performance and short-term memory have an endogenous circadian component and an evoked (e.g., sleep--wake-dependent) component. For all these parameters, the values reach their lowest point at a circadian phase corresponding to the nadir of the endogenous circadian temperature cycle. Similarly, scores are maximal at a circadian phase coinciding with the plateau phase of the endogenous circadian temperature cycle. These results suggest that an endogenous circadian pacemaker, presumably located in the suprachiasmatic nucleus of the hypothalamus, is a determinant of observed daily variations in these variables. This notion is further supported by reports that, after phase shifts induced by exposure to bright or moderately bright light, the circadian rhythms of subjective alertness and cognitive performance maintain similar temporal relations to other reliable circadian markers. Moreover, experimental manipulation of the timing of the sleep--wake cycle in healthy volunteers revealed that sleep latency, sleep efficiency and REM sleep propensity depend mainly on circadian phase, whereas the amount of SWS and slow-wave activity (SWA) depend mainly on the duration of prior waking. SWS is the amount of Stage 3 and Stage 4 sleep measured by visual inspection based on a standardized scoring method, whereas SWA is the power density in the slow range (0.75-4.5 Hz) determined digitally via a Fast Fourier transformation of the electroencephalograph (EEG) signal. Bright light exposure early in the morning can advance the time of appearance of REM sleep periods and reduce the duration of sleep episodes, whereas bright light exposure late in the evening can increase sleep latency and delay the appearance of REM sleep. These results are consistent with a phase shift in the underlying endogenous circadian pacemaker that modulates these sleep parameters.
Several groups have reported evidence that these processes may also modulate mood in healthy individuals. For instance, a diurnal variation in subjective measures of mood in healthy volunteers has been reported. Monk et al, using the constant routine protocol, reported that subjective measures of mood reached their lowest values close to the minimum of the core body temperature cycle. However, sleep deprivation itself has been shown to result in a deterioration of mood in healthy subjects, and sleep deprivation is an unavoidable limitation of the constant routine procedure. Totterdell et al and Taub et al reported, in sleep displacement studies, that the timing of sleep had a significant impact on daily mean values of mood in healthy volunteers. However, in these earlier studies, the effects of extended wakefulness and circadian phase remained confounded. Given the deterioration of subjective mood rating with sleep deprivation in healthy subjects, an experimental protocol that separates the relative influences of time elapsed since awakening from circadian phase must be used to measure this interaction. In a recent collaborative research project, we used a "forced desynchrony protocol," which is adapted from the pioneering studies by Kleitman and Kleitman. In this study, 24 healthy young subjects (16 male, 8 female) spent between 19 and 33 days on a 28-hour or 30-hour sleep--wake schedule. Because entrainment of the endogenous circadian pacemaker to days much longer than 24 hours is not possible, these conditions induce desynchrony between the circadian timing system (which continues to oscillate according to its nearly 24-hour intrinsic period) and the imposed sleep--wake cycle. Subjective mood could then be assessed at a variety of circadian phases and times since waking. Subjective mood was assessed by 2 types of visual analogue scale administered either twice every 2 hours or 3 times per hour throughout all waking episodes. In the laboratory, subjects lived individually or in groups of 2 or 3 without knowledge of the time of day for several consecutive weeks. They maintained social contacts with staff members trained to avoid communicating the time of day and the nature of the experimental conditions.
Under these conditions, a significant variation in mood was observed with circadian phase. Subjective mood declined with the descending limb of the circadian temperature curve and reached its lowest value at a circadian phase corresponding to the nadir of the temperature cycle, which under entrained conditions would occur around 5:00 am to 07:00 am (Fig. 1, left panels). Subjective mood then improved with the ascending limb of the endogenous circadian component of the temperature cycle and reached its peak around 220 to 240 circadian degrees, which under entrained conditions would occur around 10:00 pm. A statistically significant interaction of circadian and wake-dependent fluctuations was also evident. These results indicated for the first time that, in healthy young subjects, subjective mood is influenced by a complex and nonadditive interaction of circadian phase and duration of prior wakefulness. The nature of this interaction is such that moderate changes in the timing of the sleep--wake cycle may have significant effects on subsequent mood. These results indicate that the temporal alignment between the sleep--wake cycle and the endogenous circadian rhythms affects self-assessment of mood in healthy subjects. Under normal entrained conditions, subjects wake up approximately 1 to 2 hours after the endogenous minimum of the core body temperature rhythm, which occurs around 6:00 am. After 8 hours of wakefulness, the circadian phase is close to 120 to 130 degrees, a situation under which, according to the present analyses, means levels of mood will be high during the waking day.
When sleep is displaced, as it is in shiftworkers, the phase relation between the sleep--wake cycle and the endogenous circadian pacemaker changes. This change may alter mood while awake. A high prevalence of anxious and depressive symptoms has been reported among shiftworkers; our findings raise the question of whether a misalignment between the sleep--wake cycle and the endogenous circadian timing system may contribute to mood changes in that population. Our data also imply that the effects of sleep deprivation, sleep displacement and the diurnal variation of mood can no longer be attributed to a single process but need to be interpreted as a function of the simultaneous changes in circadian phase and earlier sleep--wake history. Further studies are thus needed to clarify how circadian and wake-dependent processes specifically interact in the regulation of sleep and mood in psychiatric disorders.
Reflection Exercise #7
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