Normal rhythm
A circadian rhythm is an entrainable, endogenous, biological activity that has a period of roughly twenty-four hours. This internal time-keeping mechanism is centralized in the suprachiasmatic nucleus (SCN) of humans, and allows for the internal physiological mechanisms underlying sleep and alertness to become synchronized to external environmental cues, like the light-dark cycle. The SCN also sends signals to peripheral clocks in other organs, like the liver, to control processes such as glucose metabolism. Although these rhythms will persist in constant light or dark conditions, different Zeitgebers (time givers such as the light-dark cycle) give context to the clock and allow it to entrain and regulate expression of physiological processes to adjust to the changing environment. Genes that help control light-induced entrainment include positive regulators BMAL1 and CLOCK and negative regulators PER1 and CRY. A full circadian cycle can be described as a twenty-four hour circadian day, where circadian time zero (CT 0) marks the beginning of a subjective day for an organism and CT 12 marks the start of subjective night.Humans with regular circadian function have been shown to maintain regular sleep schedules, regulate daily rhythms in hormone secretion, and sustain oscillations in core body temperature. Even in the absence of Zeitgebers, humans will continue to maintain a roughly 24-hour rhythm in these biological activities.
Circadian rhythm sleep disorders (CRSD), also known as circadian rhythm sleep-wake disorders (CRSWD), are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be caused either by dysfunction in ones biological clock system, or by misalignment between ones endogenous oscillator and externally imposed cues. As a result of this mismatch, those affected by circadian rhythm sleep disorders have a tendency to fall asleep at unconventional time points in the day. These occurrences often lead to recurring instances of disturbed rest, where individuals affected by the disorder are unable to go to sleep and awaken at "normal" times for work, school, and other social obligations. Delayed sleep phase disorder, advanced sleep phase disorder, non-24-hour sleep–wake disorder and irregular sleep–wake rhythm disorder represents the four main types of CRSD. Overview
Humans, like most living organisms, have various biological rhythms. These biological clocks control processes that fluctuate daily (e.g., body temperature, alertness, hormone secretion), generating circadian rhythms. Among these physiological characteristics, the sleep-wake propensity can also be considered one of the daily rhythms regulated by the biological clock system. Humans sleeping cycles are tightly regulated by a series of circadian processes working in tandem, allowing for the experience of moments of consolidated sleep during the night and a long wakeful moment during the day. Conversely, disruptions to these processes and the communication pathways between them can lead to problems in sleeping patterns, which are collectively referred to as circadian rhythm sleep disorders.