Introduction to Circadian Rhythm Disorders

Abstract

Circadian rhythm sleep/wake disorders (CRSWDs) encompass a wide variety of maladies in which there is misalignment of the endogenous circadian rhythm and the light/dark cycle. They are thought to occur because either (a) the external environment is not properly synchronized with the internal circadian system (e.g., jet lag, shift work) or (b) the circadian system itself is misaligned or malfunctional with respect to the external environment (e.g., delayed sleep phase, advanced sleep phase, and others). At least 3{\%} of individuals may suffer from CRSWDs, but this number may be as high as 10{\%} due to a missed diagnosis. The International Classification of Sleep Disorders third edition (ICSD-3) includes six types of circadian rhythm sleep/wake disorders: advanced sleep phase type, delayed sleep phase type, irregular sleep/wake type, free-running type, jet lag type, and shift work type. The clinical presentation of CRSWDs is influenced by numerous environmental, physiologic, psychologic, and social factors. This chapter will introduce several key concepts germane to the understanding of CRSWDs, including a description of how circadian rhythms are normally controlled and the underlying neuroanatomy and molecular biology. This circadian system controlling sleep has three components: the circadian oscillator in the suprachiasmatic nucleus (SCN, approximating a rhythm of 24.18 hours), input pathways for other external/environmental stimuli (primarily light) to synchronize the SCN, and finally the output pathways from the SCN. The sleep/wake cycle has two theoretical primary influences, a longitudinal homeostatic drive, termed ``Process S,'' and a circadian drive, termed ``Process C.'' This chapter will examine how the circadian cycle can be influenced by several internal and environmental cues including melatonin and light. Evaluation of CRSWDs can be done through a variety of measures, including chronotype questionnaires and actigraphic monitoring. General consideration for treatment will also be reviewed.