Ripple(NREM) and ripple(sp) density were not significantly different from each other (Figure 1) in the ANT (t14 = 1.427, p = 0.176) but a significant difference was found in the MD (t9 = 2.293,p = 0.048), where the ripple(sp) density was larger than the ripple(NREM) density, indicating the more frequent coupling of ripples to sleep spindles compared to spindle-free NREM periods.