Humans spend around 30% of their lives sleeping, and for this reason, the sole researcher Allan Rechtschaffen wrote that “If sleep doesn't serve an absolutely vital function, it is the greatest mistake evolution ever made”. In this aspect, sleep is an active, reversible phenomenon, and characterized by reduced vital functionsSIEGEL, 2008differentiated into two main ones: REMRapid Eyes Movement) and NREMNon-Rapid Eyes Movement)). In addition, NREM sleep is divided into 3 stages: N1, N2 and N3 (slow wave sleep). Throughout evolution ircuit that is under the control of the suprachiasmatic nucleus (SCN) located in the hypothalamus, which is considered the body's central clock.
Thirty years ago, Borbély proposed a model of sleep regulation through two processes: homeostatic and circadiaBORBÉLY, 1982; BORBÉLY; DAAN; WIRZ‐JUSTICE; DEBOER, 2016This model strongly impacted several researches related to sleep and biological rhythms, and currently it still remains dominant in explaining how sleep regulation occurs. Therefore, this model will be used in this
Homeostatic process (S)
The author describes that the homeostatic process refers to the accumulation of fatigue throughout the day. Thus, the higher the level of fatigue, the greater the propensity to sleep. Furthermore, slow wave sleep is the main feature of the S process during sleep, while theta wave activity is a marker of this process during wakefulnessBORB; ACHERMANN, 1999).
Circadian process (C)
On the other hand, the author describes that the circadian process refers to factors that cannot be voluntarily controlled, such as the endogenous and rhythmic regulation of body functions. As an example, the fluctuation of core temperature values throughout the day and the secretion of melatonin. When the process C curve reaches its highest point, there is a greater propensity to wakefulness and, on the other hand, when it reaches its lowest point, there is a greater propensity to sleep
As mentioned earlier, the sleep-wake cycle is controlled by the suprachiasmatic nucleus, which receives light information through the retinohypothalamic tract. Briefly, this regulation within the SCN occurs through the CLOCK and BMAL1 genes, and the PER and CRY proteins. In the cell nucleus, the CLOCK and BMAL1 genes stimulate the synthesis of PER and CRY proteins, which are directed to the cytoplasm and, after being phosphorylated, are sent back to the nucleus. There, these proteins will inhibit the action of CLOCK and BMAL1 through negative feedback. This looping lasts approximately 24 hours and these genes are responsible for regulating all organic functions during the light-dark cyclePEREIRA; TUFIK; PEDRAZZOLI, 2009).
Bidirectional relationship between C and S processes
The two processes are independently regulated, but despite that, they are complementary and directly related. Specifically, the greater the distance between the C and S process curves, the greater the propensity to sleep. That is, in other words, the greatest propensity to sleep occurs when the level of fatigue is high (characterized by process S) and the projection of signals for the maintenance of wakefulness is low (characterized by process C). The figure below illustrates this relationship
Studies with sleep deprivation show both circadian components and homeostatic componentsGOEL; BASNER; RAO; DINGES, 2013). In these studies, cognitive performance, characterized by reaction time, does not increase linearly as sleep deprivation progresses. Instead, improvements and losses occur throughout the day, due to the influence of circadian factors.
Model applications
Our society has transformed the relationship with work after the advent of electric energy. Thus, we live today in a society that works 24/7 (called 24/7 society) and that, in order to be able to carry out all obligations and responsibilities, we have less and less time to sleep and rest. In this aspect, some individuals need to carry out their work activities during the dark period of the day when, in fact, they should sleep at this time. Thus, performance tends to be reduced at this time, and the probability of accidents and the development of diseases (due to the reduction of the body's immunological defenses) tends to be higherFOLKARD; TUCKER, 2003; SILVA; GUERREIRO; ANDRADE; STIELER et al., 2020Therefore, these workers are at high risk for accidents and this must be taken into account when organizing the work schedule, as well as the structuring of the entire environment (lighting, temperature, food, and evaluation of health regarding sleep disorders and life habits).
On the other hand, similar misalignment is experienced by individuals who, when traveling, cross more than 3 time zones. Symptoms include reduced cognitive performance, gastrointestinal disturbances, excessive daytime sleepiness, insomnia, and nighttime awakenings. This is due to the misalignment between the circadian process of the place of origin and destination in relation to the homeostatic process, whose adaptation occurs at a rate of approximately 1 day for each spindle crossedWATERHOUSE; REILLY; ATKINSON; EDWARDS, 2007).
Sleep pattern assessment
Actigraphy is a non-invasive technique that can be used to assess sleep pattern and sleep-wake rhythm. The actigraph is a wristwatch-like device that contains light, motion, and temperature sensors. Through this information, it is possible to extract data regarding variables such as total sleep time, latency to sleep onset, time awake after sleep onset, sleep efficiency, time awake and variables related to rhythm, such as cosinor, spectrogram, periodogram, and non-parametric variables such as L5, M10, IS and IV.
