Description of REM. A: Proportion of REM in REM bursts differed across the recording period. In addition, the proportion was lower during REM sleep than NREM sleep (* The proportion was lower at ZT2 than at ZT8-ZT12 and ZT18-ZT22 (p < 0.05); # The proportion was higher at ZT20 than at ZT5 and ZT13 (p < 0.05)). B: Number of REM in REM bursts. REM bursts during NREM sleep tended to include a higher number of REM than during REM sleep. C: Distribution of all recorded REM durations. REM duration was computed as the difference in milliseconds between the start and the end of the REM. D: Distribution of all recorded REM amplitudes. REM amplitude was computed as absolute difference from zero at the peak of the REM in mV. E: REM density in REM sleep and NREM sleep differed across the recording period. REM density during NREM sleep showed increased density at the beginning of the dark period. (* REM sleep REM density at ZT16 was significantly lower than ZT4 to ZT6; ZT10 to ZT14, and ZT18 to ZT22; # REM sleep REM density was significantly lower at ZT2 than ZT10, ZT12, and ZT18 to ZT22; + REM sleep REM density was significantly lower at ZT8 than ZT12 and ZT22; § NREM sleep REM density was significantly higher at ZT14 than all other time intervals; (p < 0.05 for all comparisons)). F: REM burst density in REM sleep did not differ across the light and dark period. REM density during NREM sleep showed increased density at the beginning of the dark period (ZT14-ZT22). A REM burst was defined as at least two eye movements not faster than 200 ms apart (* NREM sleep REM burst density at ZT14 was significantly higher than at all other times (p < 0.05)).