Circadian rhythms of psychomotor vigilance, mood, and sleepiness in the ultra-short sleep/wake protocol

Abstract

Despite its advantages as a chronobiological technique, the ultra-short sleep/wake protocol remains underutilized in circadian rhythm research. The purpose of this study was to examine circadian rhythms of psychomotor vigilance (PVT), mood, and sleepiness in a sample (n=25) of healthy young adults while they adhered to a 3 h ultra-short sleep/wake protocol. The protocol involved 1 h sleep intervals in darkness followed by 2 h wake intervals in dim light, repeated for 50-55 h. A 5 min PVT test was conducted every 9 h with the standard metrics of mean reaction time (RT; RT(mean)), median RT (RT(med)), fastest 10% of responses (RT(10fast)), and reciprocal of the 10% slowest responses (1/RT(10slow)). Subjective measures of mood and sleepiness were assessed every 3 h. A cosine fit of intra-aural temperature, assessed three times per wake period, established the time of the body temperature minimum (T(min)). Mood, sleepiness, and PVT performances were expressed relative to individual means and compared across eight times of day and twelve 2 h intervals relative to T(min). Significant time-of-day and circadian patterns were demonstrated for each of the PVT metrics, as well as for mood and sleepiness. Most mood subscales exhibited significant deterioration in day 2 of the protocol without alteration of circadian pattern. However, neither sleepiness nor performance was worse on the second day of observation compared to the first day. These data provide further support for the use of the ultra-short sleep/wake protocol for measurement of circadian rhythms.

Figure 1. Laboratory observation and performance trial schedule

Key: “Ss” indicates participants (e.g., “Ss 1–3” indicates participants 1–3); “T” (followed by a number) indicates the trial number for that set of participants. Open bars are 120-min periods of wakefulness in dim light (< 30 lux); black bars indicate 60-min periods of sleep (< 1 lux). Days of experiment are listed on far-left column. Time of day is listed across the bottom. Participants entered the protocol in groups of n = 3, with each group performing a PVT trial every 9 h (6 trials per participant). The start times for participants were staggered so that ~18 performances would be obtained during each of the 8 wake periods, and that each set of performances was at a different point in the progression of trials. The intent of this scheduling was to evenly distribute any fatigue effects that may occur from multiple testing across the 24-h day.

Figure 2. Pattern of intra-aural temperature during the laboratory protocol, averaged across the 25 participants

Data are expressed as mean ± SE. Average T_min was 04:16 ± 00:23 h.

Figure 3. PVT performance metrics plotted relative to time of day and circadian phase

Data are separated by day and presented as mean ± SE. See Table 1 for statistical results and significant differences between time-points.

Figure 4. Selected POMS mood variables plotted relative to time of day and circadian phase

Data are presented as mean ± SE and separated by day. The four POMS subscales with the most robust worsening of mood on day 2 of the protocol are shown. See Table 2 for statistical test results.

Figure 5. Subjective sleepiness plotted relative to time of day and circadian phase

Data are presented as mean ± SE and separated by day to display the lack of increased sleepiness on day 2 of the protocol. See Table 2 for statistical test results.