Napping promotes inter-session habituation to emotional stimuli

Abstract

The effects of a daytime nap on inter-session habituation to aversive visual stimuli were investigated. Healthy young adult volunteers viewed repeated presentations of highly negative and emotionally neutral (but equally arousing) International Affective Picture System (IAPS) photographs during two afternoon sessions separated by 2.5h. Half of the photographs were shown at both sessions (Repeated Sets) and half differed between sessions (Novel Sets). For each stimulus presentation, evoked skin conductance response (SCR), heart-rate deceleration (HRD) and corrugator supercilii EMG response (EMG), were computed and range corrected using respective maximum session-1 responses. Following each presentation, subjects rated each photograph on dimensions of pleasantness and arousability. During the inter-session interval, Nap subjects had a 120-min polysomnographically monitored sleep opportunity, whereas Wake subjects watched a non-stimulating video. Nap and Wake subjects did not differ in their subjective ratings of photographs. However, for Repeated-Set photographs, Nap subjects demonstrated greater inter-session habituation in SCR and EMG but a trend toward lesser inter-session habituation in HRD. These group differences were absent for Novel-Set photographs. Group differences across all measures were greater for negative stimuli. Occurrence of SWS during the nap was associated with greater inter-session habituation of EMG whereas occurrence of REM was associated with lesser inter-session habituation of SCR to negative stimuli. Sleep may therefore promote emotional adjustment at the level of somatic responses. Physiological but not subjective inter-session habituation to aversive images was enhanced by a daytime nap.

Figure 1

Experimental protocol showing sessions and inter-session intervals in the Nap and Wake groups. Designations A, B and C refer to the stimulus sets (Table 2) and this example shows one of 6 alternate versions that uses stimulus set A as the Repeated Set, B as the Session-1 Novel Set and C as the Session-2 Novel Set.

Figure 2

SCR to IAPS stimuli in Nap and Sleep groups. a. Repeated Set, b. Novel Sets. Bars indicate standard error of the mean (SEM). Significance levels shown only when Group × Session interaction within a specific valence (Negative or Neutral) was significant or a trend. μS^1/2/max μS^1/2 = range-corrected SCR, **p < .01.

Figure 3

HRD to IAPS stimuli in Nap and Sleep groups. a. Repeated Set, b. Novel Sets. Bars indicate SEM. Significance levels shown only when Group × Session interaction within a specific valence was significant or a trend. Delta BPM/max Delta BPM = range-corrected HRD. BPM, beats per minute, † p < .1

Figure 4

Corrugator EMG response to IAPS stimuli in Nap and Wake groups. a. Repeated Set, b. Novel Sets. Bars indicate SEM. Significance levels shown only when Group × Session interaction within a specific valence was significant or a trend. Delta mV/max Delta mV = range-corrected EMG. *p < .05 **p < .01.

Figure 5

Sleep stages during nap associated with of inter-session habituation to Negative stimuli. a. Positive association of entry into SWS with inter-session habituation of EMG to Negative stimuli. b. Negative association of entry into REM with inter-session habituation of SCR to Negative stimuli. Delta mV/max Delta mV = normalized EMG, μS^1/2/max μS^1/2 = normalized SCR, † p < .1, **p<.01, .