Fig. 4

Sleep categorization methods. Sleep stages were distinguished from distinct characteristics of the EEG spectrogram, z-axis gyroscope (for breath detection), and heart rate. Spectral power varied across stages from A slow (10 s) oscillations between slow waves and waking during Drowsiness (DW); B highest amplitude low-frequency activity during SWS (exemplified by hot colors in low frequencies [0.5-4 Hz] of the spectrogram); C lowest amplitude high-frequency activity during REM (exemplified by dark colors in the spectrogram), and D low-amplitude high-frequency activity during quiet waking (QW), and E motion artifacts during active waking (AW). We differentiated between periods of REM with F low heart rate variability (HRV) and G high HRV (independent of changes in respiratory state—apnea [not breathing] and eupnea [breathing consistently]). We demonstrate HRV patterns due to G respiration, H independent of respiration, and I due to both respiration and movement artifacts (due to short-duration inaccuracies in automated peak detection). During active waking, motion artifacts could be caused by large breaths or active forward movement (‘galumphing’ on land or swimming in water)