Figure 1
Visual LPP ERPs for morning compared to evening conditions. Participants were exposed to an emotionally positive, neutral, or negative pictures for 400 ms. The average Cz/Pz LPP amplitude (latency range = 700–900 ms) in the evening group was larger than the morning group, and this difference was significant for the neutral and positive picture conditions. Y axis represents voltage (µV) and x axis represents time (ms).
Table 1
LPP amplitudes.
| Picture Category | Morning Group | Evening Group | ||
|---|---|---|---|---|
| Mean | SD | Mean | SD | |
| Positive Picture LPP (µV) | –0.63 | 8.36 | 3.57* | 5.74 |
| Neutral Picture LPP (µV) | –4.17 | 6.87 | 0.73** | 5.96 |
| Negative Picture LPP (µV) | 0.39 | 6.72 | 3.17 | 5.70 |
[i] Planned comparisons were carried out via independent samples t tests in order to test for differences between the morning and evening groups. These analyses showed that the LPP ERP was significantly larger in the evening group relative to the morning group for the positive and neutral pictures. Asterisks indicate significantly different from the morning group (* = p < 0.05, ** = p < 0.05).
Table 2
Picture Ratings and Reaction Times.
| Picture Category | Morning Group | Evening Group | ||
|---|---|---|---|---|
| Mean | SD | Mean | SD | |
| Positive Picture Rating | 7.05b,c | 1.21 | 7.56b,c | .97 |
| Positive Picture Reaction Time (ms) | 356.69 | 52.99 | 357.95 | 39.15 |
| Neutral Picture Rating | 5.19a,c | 0.55 | 5.32a,c | 0.54 |
| Neutral Picture Reaction Time (ms) | 359.67 | 25.61 | 356.82 | 36.14 |
| Negative Picture Rating | 2.25a,b | 0.88 | 2.11a,b | 0.91 |
| Negative Picture Reaction Time (ms) | 358.77 | 39.60 | 356.27 | 28.67 |
[i] There was a significant main effect for picture ratings F(2,166) = 489.12, p < 0.01, There was not a main effect of time of day on picture ratings or a significant picture rating × time of day interaction. Superscripts indicate significant differences across picture ratings separately for the morning and evening groups. There were no significant main or interaction effects for the reaction time ratings.
a = significantly different from positive picture ratings.
b = significantly different from neutral picture ratings.
c = significantly different from negative picture ratings (all p’s < 0.01).
Table 3
Sleep Questionnaires and Cortisol.
| Picture Category | Morning Group | Evening Group | ||
|---|---|---|---|---|
| Mean | SD | Mean | SD | |
| PSQI | 7.48 | 3.58 | 6.90 | .2.7 |
| Ave Hours Sleep | 6.72 | 1.41 | 6.71 | 2.48 |
| MEQ | 44.00 | 8.17 | 45.31 | 10.34 |
| Cortisol (µg/dl) | 0.40 | 0.35 | 0.14 | 0.08** |
[i] Table 3 shows the results for likely confounding or modifying variables. There was not a significant difference in sleep quality (PSQI) or time-of-day preference (MEQ) between groups, indicating that the LPP changes were not likely related to these factors. Follow up correlations between these sleep measures and the LPP for all pictures categories were also not significant. As expected, there was a time-of day effect of cortisol, but cortisol levels did not relate to the LPP measures for any picture category. ** indicates significantly different from the morning group.