Scientific inveStigAtionS introduction:Obstructive sleep apnea (OSA) is widely accepted to improve during slow wave sleep (SWS) compared to lighter stages of NREM sleep. However, supporting data to establish the magnitude and prevalence of this effect is lacking. Consequently, we examined this phenomenon, controlling for posture, in a large group of patients investigated for OSA at an academic clinical sleep service. Methods: A detailed retrospective analysis was conducted on data obtained from each 30-sec epoch of sleep in 253 consecutive fullnight diagnostic polysomnography studies performed over a 3-month period. Respiratory and arousal event rates were calculated within each stage of sleep, in the supine and lateral postures, and across the whole night, with OSA patients classified on the basis of an overall apnea-hypopnea index (AHI) ≥ 15 events/h. Central sleep apnea (CSA) patients were defined by a central apnea index ≥ 5/h. Sleep latency and time, and respiratory and arousal event rates in OSA, CSA, and non-OSA patients were compared between sleep stages and postures using linear mixed model analysis. The numbers of patients achieving reduced event rates in SWS and in the lateral posture were also examined.Results: There were 171 patients with OSA, 14 with CSA, and 68 non-OSA patients. OSA patients took significantly longer to achieve slow wave and REM sleep (p < 0.001) than non-OSA patients and had less stage 4 sleep (p = 0.037). There were striking improvements in AHI and arousal index (AI) from stage 1 to 4 NREM sleep (p < 0.001), with intermediate levels in REM sleep. AHI and AI were also markedly reduced in lateral versus supine sleep in all sleep stages (p < 0.001), with an effect size comparable to that of the slow wave sleep effect. The majority of OSA patients achieved low respiratory event rates in SWS. Eighty-two percent of patients achieved an AHI < 15 and 57% < 5 events/hour during stage 4 sleep. Conclusion:Although OSA patients demonstrate both a delayed and reduced proportion of SWS compared to non-OSA subjects, once they achieved SWS, AHI, and AI markedly improved in most patients.

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New methods are needed to quantify infiltration into frozen soil, an important issue for agricultural management in northern latitude regions. A dye tracer method that uses digital image analysis and fluorescence imaging is presented for visualizing and quantifying flow pathways in frozen soils. The method was applied to three soil columns in a cold chamber. Two of them were packed with sand and the third was an undisturbed soil monolith. After complete freezing to −5°C, the columns were irrigated at above‐freezing temperatures before they were vertically and horizontally sectioned to analyze pictures of the stained profiles and cross‐sections. Image analysis was done for either the visual or fluorescent spectral ranges of three different tracers. Small samples were taken from the profiles to calibrate the dye tracer concentration. This was achieved by means of second‐order polynomials of the R, G, and B values from the corresponding areas of the pictures with coefficients of determination of 0.92 to 0.99. This method results in concentration maps with a high spatial resolution reflecting the infiltration pattern. The experiment confirmed current hypotheses of infiltration mechanisms into frozen soil in that the infiltrability of the initially wet sand was restricted, whereas in the undisturbed soil monolith, the dye solution infiltrated through preferential pathways which were air filled at the time of freezing.