The circadian rhythm, commonly referred to as the human sleep cycle is the cyclical 24-hour period of human activity, specifically in reference to the individual’s sleep patterns. A normal circadian rhythm consists of a person sleeping for approximately eight hours and spending the remaining sixteen hours awake within the 24-hour day span. During the waking hours the individual’s mental and physical functions are most active as opposed to mostly lying dormant during sleep. Voluntary muscle control nearly disappears, and metabolic rate, respiration, body temperature and blood pressure is decreased during sleep. Hormones such as adrenaline, that are produced in a wakeful state are generally released in maximal amounts about two hours before awakening. The circadian rhythm is regulated by the suprachiasmatic nuclei (SCN) located in the hypothalamus. Generally the hypothalamus is regarded to be the center for integrating rhythmic information and establishing sleep patterns in the brain. The SCN receives signals via the optic nerve about the light and darkness perceived by the eye. Photoreceptive cells called intrinsically photosensitive retinal ganglion cells (ipRGCs) discovered in 1923 and comprising less than 1% of photoreceptors in the eye are mainly responsible for regulating the circadian rhythm. ipRGCs are unlike rods and cones not image-forming cells, meaning they don’t help in actually seeing. Instead they react slowly to light exposure over time and regulate melatonin production among other things. When the eye is exposed to light the ipRGC cells send signals directly to the SCN via neurons in the retinohypothalamic tract. The signals are then transmitted to the pineal gland which is responsible for the production of the hormone melatonin. The color of light detected by the photoreceptor cells also play a part in how effective the light is at preventing production of melatonin. Blue light has been proven to be most effective in slowing the production of melatonin while red light has been shown to be the least effective. Fluctuations in melatonin levels throughout the day are vital for maintaining a normal circadian rhythm. Melatonin is carried by the circulatory system making it easy for the hormone to travel throughout the brain and body. Tissues containing melatonin receptors are able to detect change in the amount of circulating melatonin and thus signal to the body and brain when it is night-time. Melatonin levels are usually measured using blood or saliva samples, revealing at what point in the circadian cycle an individual is at the time when the sample was taken. When the photoreceptors are not detecting light the body prepares for sleep, displaying physical changes associated with sleepiness such as decreased blood pressure and body temperature coming as a result of melatonin binding to receptors in the SCN. Measuring circadian rhythmicityWhen attempting to measure circadian rhythmicity in neurobehavioural variables there are various techniques that can be used. Methods for subjective measurements of alertness and fatigue such as the Visual Analogue Scale (VAS), the Stanford Sleepiness Scale (SSS), the Karolinska Sleepiness Scale (KSS), the Activation-Deactivation Adjective Check List (AD-ACL),  and the Profile Of Mood States (POMS) have largely been deemed successful. The downside of these methods is that results are difficult to control.The nature of these tests can end up masking the actual circadian rhythmicity because of non-circadian variables on the measurements of circadian rhythmicity. Common non-circadian variables that have been found to alter results in effective measures include both physical and mental disturbances. Distractions by irrelevant stimuli, boredom and motivational factors, stress, food intake, posture, ambient temperature, background noise, lighting conditions, and drug intake (e.g., caffeine) have all been shown to affect the outcome of these subjective measurements of alertness and fatigue.The alternative to subjective measures of understanding the circadian rhythm is tests of objective performance. Instead of relying on subjectivity, many studies use search-and-detection tasks and choice reaction time tasks in order to measure circadian rhythmicity. Typically the dependent variable is either the speed, accuracy or a combination of speed and accuracy of responses to repetitive stimuli. Therefore there are multiple cognitive skills that are measured in these types of tests. These skills include simple sorting, local reasoning, memory access and more complex activities such as school performance. Different tasks and different parameters for measuring how well the tasks are performed may yield different peak phases of circadian rhythmicity. Because of the varying results when measuring circadian rhythmicity it is speculated that there may be many different circadian rhythms with different phases ranging from person to person. However it has also been shown that most inter-task differences disappear under strict laboratory conditions. Under such conditions subjective methods seem to largely covary with objective methods as well suggesting that there might in fact be a baseline for a human circadian rhythm that can be altered as the surrounding environment changes. Evidence that furthermore supports this is the fact that the circadian rhythm also coverys with the core body temperature, measuring the core body temperature has thusly shown to be an effective way of roughly measuring the biological clock. Generally low temperature corresponds to good performance and low temperatures correspond to poor performance.Sleep deprivation Common in youths attending school, sleep loss and sleep deprivation causes worsened cognitive performance characterized by an increase in errors, diminished job performance in many occupations such as medical residents and shift workers. Just one night of worsened sleep can cause lapses in attention and increases risks of traffic accidents. When sleep is denied frequently deterioration of vigilant attention and cognitive performance begins. Sleep deprivation also causes decreased brain activity worsen cognition, motor skills and mood.When Individuals are repeatedly denied sleep they generally lose the ability to assess where they are alert enough to safely perform a task because of worsened cognition. The individual also loses the ability to know whether they truly need to rest, because of this individuals can get stuck in a negative spiral where their performance is worsened but they do not sleep more to repair the damage. Subjective measures of alertness plateau within a few days as opposed to cognitive performance declines across multiple weeks.More sources of deviationThe mid-afternoon dipSome individuals have been shown to experience a short term dip in fatigue, alertness and core body temperature indicating a dipping phase in their circadian rhythm. This phenomenon has been dubbed the mid-afternoon dip or the post-lunch dip. The mid-afternoon dip appears to have no relation to food intake and have been observed  in field studies as well as laboratory experiments. However this afternoon dip is not always found in all people. Whether the mid-afternoon dip is shown to exist in some form in all people or only applies to some individuals you could conclude that avoiding assessments during the mid-afternoon should ideally be avoided in order to lessen the risk that students performance is worsened because of their circadian phase.The practice effect  The practice effect, meaning the effect where cognitive performance is improved across a couple of days of testing. While it is difficult to separate the circadian rhythm from the practice effect it may be possible to lessen the effect by testing subjects at different times of day. This would make the practice effect average out across all subjects. This only works by assuming that the practice effect is cumulative with the circadian rhythm and affects every subject in the same way. It is worth noting that there have been no studies to find out whether the practice effect and circadian rhythm can in fact be canceled out using this method. The most widely used method for avoiding the practice effect is to train subjects in asymptotic performance before assessing the circadian rhythm. The importance of eliminating the practice effect is especially important not only because it may conceal, accentuate or in other ways distort results in this type of studies, the practice effect also effects many of the same variables that we use to estimate both fatigue and alertness as well as cognitive performance. This means that changes in the true circadian curve that is trying to be estimated can occur.Inter-individual differencesTypically studies reporting their results will represent a mean average of their results. In cases where inter-individual differences in the circadian period have been reported that stray away from the normal circadian period of 24-hours. Some things that have been shown to slightly alter one’s circadian rhythm include personality and age.When examining the reports of inter-individual differences a clear pattern of morningness/eveningness has been shown to be the most drastic type of variation. Morningness, meaning the tendency to be an early bird ass opposed to eveningness being the tendency to be a night owl has shown to differ greatly from person to person and appears to be biological and hereditary. This means that some people consistently will perform better in the mornings while others will be more alert in the evening.

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