FIGURE 2.

The trajectory of nocturnal heart rate (HR) across the stages of sleep throughout the night. The nocturnal HR of a good sleeper averaged over 30‐s epochs is displayed throughout the sleeping period. Sleep stages are overlaid with different colours to highlight the fluctuations in HR attached to different sleep stages throughout the night. Epochs of Stages 3 and 4 are more frequent and longer in duration during the first half of the night and are generally accompanied with dips in nocturnal HR. This graph also highlights the influence of circadian phase on nocturnal HR with the lowest points of HR occurring the mid‐point of the night emulating a cosine curve (consistent with the nadir of core body temperature; Massin et al., 2000; Nakagawa et al., 1998). Furthermore, heightened HR across epochs of rapid eye movement (REM; found primarily across the second half of the night) is observed, the timing and duration of which are generally considered to be under circadian control (Beersma & Gordijn, 2007)