Effects of 45 min versus 15 min fear extinction training on the percentage of time spent in REM sleep. These effects were observed during the 6 h sleep—wake recording period that immediately followed fear extinction training. Note the significant increase in REM sleep during the first 4h (Ext, n = 8) and 3h (Failed-ext, n = 6) of sleep—wake recording in rats that received 45 min of training and the significant decrease in REM sleep during the first 3 h of sleep—wake recording in rats that received only 15 min of training (No-ext, n = 8). Also note the significant increase and temporal differences in REM sleep in Ext rats compared with both No-ext and Failed-ext rats. Error bars represent SEM; *p < 0.05, **p < 0.01, ***p < 0.001.