FIGURE 1.

Example of the transition from awake state to non-REM sleep and later to REM sleep in a 10-year-old boy. In this spectral analysis of HRV, one can clearly see the gradual formation and increase of RSA as a band around 0.25 Hz. Although the total activity is higher during awake state, the activity in the RSA band stands out during the non-REM sleep – falling asleep qualitatively resembles a phase transition normally studied in physics (Bartsch et al., 2012; Penzel et al., 2016). This process is reversed when entering the REM stage (from minute 63 on). This figure is not a part of the study reported in this paper, but it was done independently, as part of another study reported in Bonin et al. (2004).