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The objective of this study was to assess the validity of a sleep/wake activity monitor, an energy expenditure activity monitor, and a partial-polysomnography system at measuring sleep and wake under identical conditions. Secondary aims were to evaluate the sleep/wake thresholds for each activity monitor and to compare the three devices. To achieve these aims, two nights of sleep were recorded simultaneously with polysomnography (PSG), two activity monitors, and a partial-PSG system in a sleep laboratory. Agreement with PSG was evaluated epoch by epoch and with summary measures including total sleep time (TST) and wake after sleep onset (WASO). All of the devices had high agreement rates for identifying sleep and wake, but the partial-PSG system was the best, with an agreement of 91.6% ± 5.1%. At their best thresholds, the sleep/wake monitor (medium threshold, 87.7% ± 7.6%) and the energy expenditure monitor (very low threshold, 86.8% ± 8.6%) had similarly high rates of agreement. The summary measures were similar to those determined by PSG, but the partial-PSG system provided the most consistent estimates. Although the partial-PSG system was the most accurate device, both activity monitors were also valid for sleep estimation, provided that appropriate thresholds were selected. Each device has advantages, so the primary consideration for researchers will be to determine which best suits a given research design.

SUMMARYSubjective alertness may provide some insight into reduced performance capacity under conditions suboptimal to neurobehavioural functioning, yet the accuracy of this insight remains unclear. We therefore investigated whether subjective alertness reflects the full extent of neurobehavioural impairment during the biological night when sleep is restricted. Twenty-seven young healthy males were assigned to a standard forced desynchrony (FD) protocol (n = 13; 9.33 h in bed ⁄ 28 h day) or a sleeprestricted FD protocol (n = 14; 4.67 h in bed ⁄ 28 h day). For both protocols, subjective alertness and neurobehavioural performance were measured using a visual analogue scale (VAS) and the psychomotor vigilance task (PVT), respectively; both measures were given at various combinations of prior wake and circadian phase (biological night versus biological day). Scores on both measures were standardized within individuals against their respective baseline average and standard deviation. We found that PVT performance and VAS rating deviated from their respective baseline to a similar extent during the standard protocol, yet a greater deviation was observed for PVT performance than VAS rating during the sleep-restricted protocol. The discrepancy between the two measures during the sleep-restricted protocol was particularly prominent during the biological night compared with the biological day. Thus, subjective alertness did not reflect the full extent of performance impairment when sleep was restricted, particularly during the biological night. Given that subjective alertness is often the only available information upon which performance capacity is assessed, our results suggest that sleep-restricted individuals are likely to underestimate neurobehavioural impairment, particularly during the biological night. IN TROD UCTI ONShiftwork, in particular involving night work, disrupts the normal sleep ⁄ wake pattern in humans, causing temporary deficits in neurobehavioural performance and a heightened risk of errors and accidents (Akerstedt, 1998). Performance of safety-critical tasks could be maintained at an adequate level, and errors and accidents can be reduced, however, if necessary compensatory or preventative actions, such as mobilizing mental efforts and employing countermeasures, take place (Dinges et al., 1987;Fairclough and Graham, 1999;Purnell et al., 2002). Yet the application of such actions is guided by self-knowledge of performance capacity (Fairclough and Graham, 1999). Accurate knowledge of performance capacity, therefore, is critical for productivity and safety under conditions that are suboptimal for neurobehavioural functioning.Subjective alertness is arguably the most readily accessible information upon which performance capacity is

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