ABSTRACT
Sleep is a complex phenomenon that involves various brain structures and physiological processes. As sleep deepens, two electrical phenomena begin to appear periodically, and constitute the first neurophysiological break-point in brain function before a second continuum is reached. Firstly, sleep spindles are short bursts of fast activity mainly in the sigma band and the second electrical phenomenon is the K-complex–a brief, high-voltage peak that occurs either spontaneously or in response to external stimuli. Sleep and its different stages are usually studied with electroencephalography (EEG), a technique that reveals sleep-stage-specific variations in brain activity. EEG observations in humans can distinguish between the first four stages, encompassed by the term "non-rapid eye movement sleep", and a fifth "rapid eye movement sleep" stage. Sleep onset and composition depend on circadian modulations and autonomic inputs. All the mechanisms influenced by the biological clock appear to be linked to autonomic activity, and the same is true of sleep.
