Hypnogram after R&K classification of ICU patients with corresponding IDOS tracings. The IDOS index, calculated from EEG channel C3/C4 is shown below the hypnograms after R&K analysis for all five ICU patients. Both R&K classification and the IDOS index of PSG recordings show relatively normal sleep for Patient A, particularly on the second night. The first night shows some arousals, but there is still transitioning to SWS. Nearly all sleep is seen during the night. Patient B shows severe fragmentation, with sleep spread evenly between day and night. In the hypnogram and IDOS tracing rapid switching between sleep and wake is visible. Patient C’s hypnogram shows a period of sleep in the first night, when the patient was non-sedated and mechanically ventilated. After extubation following the first night, sleep architecture seems to gradually deteriorate. Sleep on days two and three is fragmented with little SWS and no distinct sleep cycles in either tracing. Patient D shows severe fragmentation by awakenings and a period of daytime-sleep, visible in the hypnogram and IDOS tracing. Patient E shows a single ultradian sleep cycle around 2:00 in both tracings, followed by a period of fragmented sleep. All sleep occurred during the evening and night according to the hypnogram, although the IDOS tracing suggests short bursts of sleep during the day. EEG, electroencephalogram; IDOS, ICU Depth of Sleep; PSG, polysomnography; R&K, Rechtschaffen and Kales; SWS, slow wave sleep.