Figure 1. Total sleep time (a), sleep efficiency (b), mean activity score (c), and subjective sleep quality (d) at low and moderate altitude compared to sea level. The horizontal red line represents the mean of each sleep variable at sea level. Data are presented as M ± SD. Significant difference from sea level * p< 0.05. Note: Day 16 – 25 represents data collected at low altitude in Denver (1600 m), where Day 16 represents the first day at low altitude. Day 26 – 31 represents data collected at moderate altitude in Manizales (2150 m).