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. 2020 Sep 18;11:549908. doi: 10.3389/fneur.2020.549908

Figure 1.

Figure 1

Comparison of sleep and circadian rhythms in healthy adult and adult with critical illness in the ICU. In a healthy adult, the circadian clock is synced to the daily external cycle of changing light, sound, and temperature. The sleep stage of a healthy adult occurs during the night and is composed of 2–5% light sleep, 48–70% deep sleep, 20–25% rapid eye movement (REM) sleep. REM sleep reoccurs in cycles of 90–120 min. Acetylcholine is predominantly discharged during wakefulness and REM sleep; while GABAergic activity is predominant during deep sleep. Dopaminergic activity promotes alertness and reduces sleep. Melatonin secretion starts around 9.30 pm (dim light melatonin onset) and stops around 7.30 am, peaking around 3 am. Cortisol secretion starts in the early morning, peaks around 10 am in correspondence to the time of highest alertness, and keeps declining gradually throughout the day, and the night. Overall, these processes coordinate physiological functions including cardiovascular functions and temperature. In contrast, the intensive care unit (ICU) constant artificial environment disrupts the daily cycle of circadian functions. In the ICU critically ill patients, compared to healthy adults, present equal to normal sleep time in the course of 24 h but the majority of it consists of light sleep; 50% of sleep time is distributed during the day and disturbed by frequent arousals. For instance, benzodiazepine decrease sleep latency, slow wave sleep (SWS) and REM sleep duration and frequency; propofol suppresses SWS EEG bursts; opioids alter REM sleep; while, dexmedetomidine improve stage 2 and sleep efficiency by shifting 75% of sleep to nighttime. In ICU patients, the acute stress environment has been associated with decreased GABAergic and cholinergic transmission, and increased dopaminergic transmission, impaired melatonin secretion and increased cortisol production, along with displacement of physiological functions normally coordinated by the circadian clock. These disturbances have been associated with symptoms of delirium. ICU, intensive care unit; REM, rapid eye movement; GABA, gamma-aminobutyric acid.