In this review, the authors explain how human circadian rhythm has developed in the process of evolution, mechanisms of human circadian control, modification of human circadian rhythm and therapeutic principles of circadian rhythm related sleep disorders. In order to treat circadian rhythm related sleep disorders, precise understanding of the mechanism and proper use of circadian phase modifiers are necessary. Therapeutic principles involve bright light, melatonin, exercise and their phase response curves.

Background: Severe kyphoscoliosis may result in significant ventilatory defect, cardiorespiratory failure and death. Recent studies have suggested that sleep disordered breathing (SDB) may also contribute. Guilleminault and colleagues used the term “Quasimodo” syndrome for obstructive SDB in kyphoscoliosis after the famous hunchback of Notre Dame described by Victor Hugo. Contrary to these findings Sawicka and Branthwaite reported no evidence of obstructive SDB but only hypoventilation and oxygen desaturation during sleep in 21 subjects with non-paralytic and paralytic kyphoscoliosis. The authors concluded that higher body weight (57-71 Kg) and presence of systemic hypertension could have contributed to obstructive SDB in the Guilleminault study. Methods: We analyzed our data of 11 cases of idiopathic kyphoscoliosis evaluated for SDB. The cases were referred in view of complaints of exertional dyspnoea during pre operative assessment for corrective spine surgery. A detailed history included respiratory symptoms, snoring and excessive daytime sleepiness (EDS). All patients were subjected to chest and thoracic radiograph, spirometry with flow volume loop (FVL), arterial blood gas analysis (ABG) and 2- dimensional echocardiography (2-D ECHO). Cobb's angle was calculated to assess the severity of scoliosis. Level 3 sleep study in the form of night- time recording of cardiorespiratory variables. Arterial CO₂ monitoring or end tidal CO₂ could not be performed. Titration study was performed with continuous positive airway pressure (CPAP) and bi-level positive airway pressure (PAP) on 2 separate nights. Results: Eleven patients, age range 13-50 years, 4 men and 7 women, bodyweight 21-43 Kg with kyphoscoliosis were studied. All had exertional dyspnoea, 3 complained of EDS while none snored. Four had previous history suggestive of right heart failure i.e. episodes of puffiness of face and pedal oedema. Cobb's angle was greater than 60 (70-110) degrees in all cases. All 11 cases had severe restrictive abnormality on spirometry and awake compensated type 2 respiratory failure i.e. hypoxaemia, hypercapnoea with a normal pH on arterial blood gas analysis. Four cases with symptoms of right heart failure were confirmed to have pulmonary hypertension and cor pulmonale on 2-D ECHO. Level 3 sleep study using cardiorespiratory variables showed nocturnal desaturations in one case while remaining 10 cases showed SDB in the form of hypoventilation and oxygen desaturations (hypoventilation/ hypoxemia syndrome associated with sleep). While CPAP titration showed no improvement, titration study performed with bi-level positive airway pressure (PAP) showed improvement in hypoventilation and/or oxygen saturation. All cases were treated with bi-level therapy and showed improvement in symptoms and daytime arterial blood gases.