Changes of heart rate and high frequency, percentage of low frequency and ratio of low frequency to high frequency components in three conditions. The vertical line represents the standard error. *indicates the significant level after Friedman test; P <0.05. †indicates the significant level after Scheffe’s post-hoc test; P <0.05. Reprinted from Okamoto-Mizuno K, Tsuzuki K, Mizuno K, Ohshiro Y: Effects of low ambient temperature on heart rate variability during sleep in humans. Eur J Appl Physiol 2009, 105:191–197 with kind permission from Springer Science and Business Media.