Living Without Temporal Cues: A Case Study

Abstract

Isolation from external time cues allows endogenous circadian rhythmicity to be demonstrated. In this study, also filmed as a television documentary, we assessed rhythmic changes in a healthy man time isolated in a bunker for 9 days/nights. During this period the lighting conditions were varied between: (1) self-selected light/dark cycle, (2) constant dim light, and (3) light/dark cycle with early wake up. A range of variables was assessed and related to the sleep-wake cycle, psychomotor and physical performance and clock-time estimation. This case study using modern non-invasive monitoring techniques emphasizes how different physiological circadian rhythms persist in temporal isolation under constant dim light conditions with different waveforms, free-running with a period (τ) between 24 and 25 h. In addition, a significant correlation between time estimation and mid-sleep time, a proxy for circadian phase, was demonstrated.

Keywords: circadian; clock-time estimation; dim light; mid-sleep; performance; phase advanced; time isolation; wrist temperature.

FIGURE 1

Experimental Design. (A) Protocol of the experiment with the duration of each stage (57 h, Self-selected Light/dark cycle; 107 h, Constant Dim Light; and 52 h, Light/dark cycle with early wake up), (B) variables measured continuously throughout the experiment, (C) diagram of tests performed and the seven approximate times (blue arrows) within each waking period.

FIGURE 2

Ten-day ambulatory recording of (A) time in movement (light green) and motor activity (dark green), (B) wrist temperature (red) and subjective sleepiness (Karolinska Sleepiness Scale, KSS) (blue), (C) sleep probability (black) and light exposure (orange). Gray areas indicate Stage 2 (constant dim light). Previous and subsequent days correspond to Stage 1 (self-selected light/dark cycle) and Stage 3 (light/dark cycle with early wake up), respectively. Black bulb indicates darkness, yellow bulb indicates bright light (Stage 1, self-selected light/dark cycle), gray bulb indicates dim light (Stage 2) and alarm clock indicates light/dark cycle with early wake up (Stage 3).

FIGURE 3

Actogram built from the daily records showing sleep periods in yellow and mid-sleep time (red circles) for each night. Black bulb indicates darkness, yellow bulb indicates bright light (Stage 1, self-selected light/dark cycle), gray bulb indicates dim light (Stage 2) and alarm clock indicates light/dark cycle with early wake up (Stage 3).

FIGURE 4

Ten-day ambulatory recordings of wrist temperature rhythm (red) plotted with the difference between actual and estimated time since wake up (black, A). Daily mean difference (± SD) between actual and estimated time since wake up (B, solid line, black) with mid-sleep time (dashed line, blue). Gray area indicates Stage 2 (constant dim light). Previous and subsequent days correspond to Stage 1 (self-selected light/dark cycle) and Stage 3 (light/dark cycle with early wake up), respectively.

FIGURE 5

Ten-day ambulatory recordings of wrist temperature rhythm (red) plotted with reaction time (A, blue), digit symbol substitution test (DSST) (B, green), grip strength (C, brown), sprint time (D, green), mood (higher values, more depressive mood) (E, turquoise). Gray areas indicate Stage 2 (constant dim light). Previous and subsequent days correspond to Stage 1 (self-selected light/dark cycle) and Stage 3 (light/dark cycle with early wake up), respectively. Black bulb indicates darkness, yellow bulb indicates bright light (Stage 1, self-selected light/dark cycle), gray bulb indicates dim light (Stage 2) and alarm clock indicates light/dark cycle with early wake up (Stage 3).