Figure 1
Response of equine circadian melatonin (MT) and body temperature (BT) rhythms to a 6-h phase advance of LD12:12. The LD cycle with 6-h phase advance is depicted above each graph with white bars representing light and black bars times of darkness. The abscissa represents light cycle time (ZT) in hours, where ZT 0 corresponds to lights on and ZT 12 to lights off of a 12 h photoperiod. Through Day 0 (curves at left) and for one additional day, lights were on 7:30 AM to 7:30 PM (ZT0-12). As indicated by the arrow, the photoperiod was advanced 6 h on Day1 (+Δφ = 6 h, from ZT18 to ZT 0) to give new lights on from 1:30 AM to 1:30 PM (Days 1–11). Baseline curves (mean+/-SEM) for MT (top) and BT (bottom) are plotted on the left from ZT 0 to ZT 24. Curves for Day 1 through 11 are plotted on the right with point symbols and SEM removed for clarity. The dotted lines retrace the Day 0 curve assuming no phase shift. Asterisks represent Day 0 mean acrophase times.
Table 1
MT and BT mean mesor, amplitude and acrophase values
| Melatonin | Temperature | |||||
| Day | Mesor (pg/mL) | Amplitude (pg) | Acrophase (CT) | Mesor (°F) | Amplitude (°F) | Acrophase (CT) |
| 0 | 4.61 | 5.66 | 1:35 | 100.63 | 0.45 | 12:01 |
| 1 | 5.7 | 8.45 | 8:36 | 100.55 | 0.45 | 8:33 |
| 3 | 4.16 | 5.13 | 8:50 | 100.45 | 0.4 | 6:07 |
| 5 | 5.63 | 7.38 | 8:08 | 100.57 | 0.38 | 6:00 |
| 7 | 4.35 | 6.0 | 8:31 | 100.23 | 0.45 | 7:58 |
| 9 | 4.23 | 5.65 | 8:45 | 100.37 | 0.28 | 6:34 |
| 11 | 4.46 | 5.78 | 7:57 | 100.38 | 0.36 | 6:37 |
Values are from baseline Day 0 and Days 1–11 post 6-h phase advance of LD cycle.
Figure 2
Time course of reentrainment of MT (left) and BT (right) rhythms to a 6-h phase advance of LD12:12. The Figure 1 curves for Days 1 through Day 11 are re-plotted in a vertical array to show mean+/-SEM in relation to the Day 0 curve, which has been advanced 6 h for comparison. Dotted lines connect the upper and lower 95% confidence limits of the phase shifted Day 0 means. Without exception, Day 1 through Day 11 melatonin curves closely paralleled the zeitgeber time adjusted (6 h advanced) baseline Day 0 curve. By contrast, BT curves on Days 7, 9 and 11, deviate significantly from baseline at individual ZT times marked by adjacent black asterisks (* p < .05; ** p < .01; *** p < .001). Large red asterisks beneath each curve represent cosine fitted acrophases (ZT), which for BT, are notably phase delayed in association with the waveform distortions on Days 7–11 (see Figure 3A). Other conventions as in Figure 1.
Figure 3
Time course of changes in cosine acrophase, amplitude and mesor for equine MT and BT rhythms with a 6-h phase advance of LD12:12 between Day 0 and Day 1. A. Change in acrophase clock time (mean +/- SEM) over Days 0 through 11 of the experiment. B. mean +/- SEM melatonin amplitude and mesor values. C. Means +/- SEM body temperature amplitude and mesor (right y axis) (Range in degrees centigrade: 37.8°C – 38.9°C). Dotted horizontal lines in A represent a 6 h phase advance relative to baseline (upper, MT; lower BT). Note that the consistent steep rise in MT between ZT 12 and 15 on all days suggests a rapid MT shift that is essentially complete (+ 6 h) on Day 1 and stable thereafter (Figure 2A).