Circadian process

Sleep is thought to be under the control of two independent processes one homeostatic (process S) and the other circadian (process C), which together... 

Figure 7.5 Histograms illustrating typical behavioral state changes observed following bilateral lesions of dopaminergic ventral tegmental pathways in rats receiving 6-hydroxydopamine into the nucleus accumbens (217). Notable amounts of REM sleep are evident during both the major wake (1900-0700) and major sleep (0700-1900) periods. Maintenance of the rest and activity periods to the 12 12 h light dark schedule, respectively, demonstrates the relative preservation of circadian processes.

Circadian process and AIM. The circadian ( 24 hr) curve of body temperature is shown in its entrained mode to illustrate the combined effects of light and temperature on AIM. At the zenith of the temperature curve waking is favored while following the nadir, REM sleep is favored. 

The results from the studies described above indicate that the MSLT provides a robust measure of sleepiness that is sensitive to sleep restriction in children and adolescents (112). Patterns of sleep latency from repeated naps during constant routine protocols or forced desynchrony protocols in older children and adolescents also illustrate the influences of both homeostatic and circadian processes on sleepiness/alertness and provide evidence for the hypothesis that... 

The circadian process C is implemented by a simple cosine function with period t of 1 day (1 day = 1440 min), a phase shift (tphase) and an arbitrary amplitude (amp) and base line shift (base). 

The time of falling asleep is determined by three factors, which in normal sleepers occur at bedtime. These are (a) circadian rhythm, i.e. the body s natural clock in the hypothalamus triggers the rest/ sleep part of the sleep-wake cycle, (b) tiredness, i.e. time since last sleep, usually about 16 hours and (c) lowered mental and physical arousal. If one of these processes is disrupted then sleep initiation is difficult, and it is these three factors that are addressed by a standard sleep hygiene program (see below). Early in the course of insomnia rigorous adherence to sleep hygiene principles alone may restore the premorbid sleep pattern but in some patients the circadian process is less stable and they are less susceptible to these measures. 

FIGURE 3.1. Circadian Variations in Oral Temperatures and Alertness for Six Process Workers (Monk and Embrey, 1981). 

The practical implications of this experiment are that when evaluating the effects of shift work due to circadian effects, the type of task being carried out by the worker must be taken into account. For example, skill-based tasks would be expected to exhibit the performance changes characteristic of low memory load tasks, whereas performance variations in knowledge-based tasks would be expected to follow the pattern of high memory load tasks. Performance on rule-based tasks may depend on the degree of frequency of use of the rules, which in turn may determine the memory load. If these results were confirmed by further process plant studies, it would have implications for when different types of operation (involving different levels of memory load) should be scheduled to reduce circadian rhythm effects and minimize errors. 

FIGURE 3.3. Circadian Variations in Errors Made by Process Workers... 

Circadian Rhythms. Figure 1 Mammalian circadian system is organized as a hierarchy of multiple clocks thatfunction to synchronize timing of internal processes between each other and with the environment. See details in the text. 

At its most fundamental level, the circadian cycle rests on the influence of so-called clock genes . These genes have been studied most extensively in insects but they have also been found in humans. Their protein products enter the cell nucleus and regulate their own transcription. This feedback process is linked to exposure to light and so it is not surprising that visual inputs are important for maintenance of circadian rhythms. However, it is not the reception of specific visual information, transmitted in the optic nerve to the lateral geniculate nucleus (LGN) and visual cortex (i.e. visual discrimination), that is responsible for the rhythm but the more simple, almost subconscious, reception of light. 

POA WSNs also initiates thermolytic autonomic processes with resulting heat loss. It is reasonable to speculate that the activation of these neurons underlies the peripheral vasodilation and evoked lowering of body temperature at sleep onset (reviewed by Heller, 2005) and the association of high sleep propensity with the low-body-temperature phase of the circadian temperature cycle... 

Perturbation of the 5-HT system can elicit changes in a wide variety of behaviors. Furthermore, drugs that act on serotonergic neurons and their receptors are used to treat diseases such as depression, anxiety disorders and schizophrenia. Thus, 5-HT has been implicated in the regulation of many behaviors and physiological processes. The involvement of 5-HT in three areas - neuroendocrine function, circadian rhythms and feeding behavior - will be highlighted for illustrative purposes. 

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