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For SWA good agreements were observed
For SWA, good agreements were observed for TST, WASO and SE at 17°C and 22°C. WASO was overestimated, and TST and SE were underestimated at 29°C. SOL was significantly underestimated at 17°C and 22°, although it was not statistically significantly different from PSG at 29°C. Similar to a previous study reporting a bias of −8.7min [9], the present study reported mean biases for SOL from −7.4 to −13.7min (Table 2). Although SWA employed a skin temperature sensor and a heat flux sensor to improve the accuracy of estimating SOL, significant underestimations were observed at 17°C and 22°C (p<.05) (Table 1). The mechanism for this underestimation is unclear; presumably, at the lower temperatures (17°C and 22°C), skin warming over time, due to heat trapped under bedding, may cause vasodilation and may have led to earlier sleep onset detection. SWA showed a significantly higher WASO than PSG as well as larger LoA at 29°C (Tables 1 and 2), in line with groups of outliers observed in the B–A plot (Fig. 1F). This discordance may reflect the principles of measurements and associated algorithms, in that SWA detects sleep based on skin temperature, heat flux, skin conductance and movement. A study revealed that decreases in skin conductance provided a sensitive marker for autonomic arousal during sleep [18]. It may be speculated that an increase in WASO at high ambient temperatures was associated with a decrease in skin conductance. Accordingly, additional analysis was performed to see whether galvanic skin response values during wake bouts were different at the three ambient temperatures. ANOVA revealed non-significant findings suggesting that the outliers displayed by WASO at 29°C could not be explained by a drop in galvanic skin conductance. Should the manufacturer’s algorithm be known, then discordance would be explained and accuracy could be improved. Interestingly, we recorded a higher EEG arousal rate at 22°C and 29°C than at 17°C, although the difference was not statistically significant. Notably, hot exposure increased wake time during the sleep formyl [19]. However, we cannot be ascertained of this explanation, since the SWA’s sleep–wake detection algorithm is not accessible. This higher WASO at 29°C may reflect a significantly lower TST and SE at the same temperature (Tables 1 and 2, Fig. 1G and H). Similar to AW2, SWA also showed good sleep detection (93%) but poor wake detection (57%), in agreement with a previous validation study [10]. The good sleep epoch but poorer wake epoch detection may be explained by the greater amount of time spent immobile, so that both AW2 or SWA detect sleep with greater ease resulting in high sensitivity [2,10,20]. The kappa coefficient adjusts the amount of agreement that can be expected by chance. Since a high proportion of sleep epochs occur during the sleep period, this correction for chance may have led to a relatively lower kappa statistic, compared to percentage agreement [16]. An analytic limitation was that 30-s epochs were recorded for PSG and AW2, whereas 1-min epochs were recorded for SWA. Hence, in the sleep–wake epoch analysis, we divided the SWA outputs to match each 30-s epoch [10] set for PSG and AW2. This methodology was likely biased to show poor ability of the SWA to detect sleep or wake epochs. In addition, SWA was placed on the non-dominant arm for comparison purposes. However, placing the SWA on the left arm may not yield equivalent results, given that the manufacturer-driven study compared seven subjects that wore the SWA on the right arm with two subjects that wore it on the left arm. This issue may cause discordance with PSG. In summary, AW2 showed minimal bias for the measurements of SOL, TST and SE at all three temperatures, but significantly overestimated WASO at 17°C and 22°C. SWA also showed minimal bias for WASO, TST and SE but severely underestimated SOL at 17°C and 22°C. In addition, SWA significantly overestimated WASO and underestimated TST and SE at 29°C. Wake detection cannot be ascertained under all temperature conditions for both devices. In conclusion, the results of this study show similar validity for sleep detection of SWA and AW2, except that SWA is dependent on ambient temperature. Unlike sleep studies conducted in sleep clinics, home sleep studies are conducted under more variable temperature conditions. Hence, given the current findings, monitoring of bedroom temperature is considered crucial in home sleep studies. Future studies are instigated to evaluate the concordance rates in sleep assessment for patients with delayed sleep onset, higher level of WASO and lower SE (for example, insomnia patients who have difficulty initiating/maintaining sleep, or obstructive sleep apnea patients who have fragmented sleep).